Move 'where' to parser keywords; Add 'filter' command (#7365)

# Description

This PR moves the `where` command to a parser keyword. While it still
uses the shape-directed parsing dictated by the signature, we're free to
change the parsing code now to a custom one once we remove the syntax
shapes.

As a side effect, the `where -b` flag was removed and its functionality
has moved to the new `filter` command.

Just FYI, other commands that take row conditions:
- `take until`
- `take while`
- `skip until`
- `skip while`
- `any`
- `all`

We can either move these to the parser as well or make them accept a
closure instead of row condition.

# User-Facing Changes

New `filter` command which replaces `where -b` functionality.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/src/parser.rs

@@ -17,7 +17,8 @@ use nu_protocol::{
 
 use crate::parse_keywords::{
     parse_alias, parse_def, parse_def_predecl, parse_export_in_block, parse_extern, parse_for,
-    parse_hide, parse_let, parse_module, parse_overlay, parse_source, parse_use,
+    parse_hide, parse_let, parse_module, parse_overlay, parse_source, parse_use, parse_where,
+    parse_where_expr,
 };
 
 use itertools::Itertools;
@@ -1892,6 +1893,7 @@ pub fn parse_full_cell_path(
     span: Span,
     expand_aliases_denylist: &[usize],
 ) -> (Expression, Option<ParseError>) {
+    trace!("parsing: full cell path");
     let full_cell_span = span;
     let source = working_set.get_span_contents(span);
     let mut error = None;
@@ -1996,10 +1998,11 @@ pub fn parse_full_cell_path(
 
             (out, true)
         } else if let Some(var_id) = implicit_head {
+            trace!("parsing: implicit head of full cell path");
             (
                 Expression {
                     expr: Expr::Var(var_id),
-                    span: Span::new(0, 0),
+                    span: head.span,
                     ty: Type::Any,
                     custom_completion: None,
                 },
@@ -3035,6 +3038,7 @@ pub fn expand_to_cell_path(
     var_id: VarId,
     expand_aliases_denylist: &[usize],
 ) {
+    trace!("parsing: expanding to cell path");
     if let Expression {
         expr: Expr::String(_),
         span,
@@ -4589,6 +4593,8 @@ pub fn parse_math_expression(
     lhs_row_var_id: Option<VarId>,
     expand_aliases_denylist: &[usize],
 ) -> (Expression, Option<ParseError>) {
+    trace!("parsing: math expression");
+
     // As the expr_stack grows, we increase the required precedence to grow larger
     // If, at any time, the operator we're looking at is the same or lower precedence
     // of what is in the expression stack, we collapse the expression stack.
@@ -4765,6 +4771,8 @@ pub fn parse_expression(
     expand_aliases_denylist: &[usize],
     is_subexpression: bool,
 ) -> (Expression, Option<ParseError>) {
+    trace!("parsing: expression");
+
     let mut pos = 0;
     let mut shorthand = vec![];
 
@@ -5014,6 +5022,7 @@ pub fn parse_expression(
                     spans[0],
                 )),
             ),
+            b"where" => parse_where_expr(working_set, &spans[pos..], expand_aliases_denylist),
             #[cfg(feature = "plugin")]
             b"register" => (
                 parse_call(
@@ -5149,6 +5158,7 @@ pub fn parse_builtin_commands(
         }
         b"export" => parse_export_in_block(working_set, lite_command, expand_aliases_denylist),
         b"hide" => parse_hide(working_set, &lite_command.parts, expand_aliases_denylist),
+        b"where" => parse_where(working_set, &lite_command.parts, expand_aliases_denylist),
         #[cfg(feature = "plugin")]
         b"register" => parse_register(working_set, &lite_command.parts, expand_aliases_denylist),
         _ => {
@@ -5294,6 +5304,7 @@ pub fn parse_block(
                     .iter()
                     .map(|command| match command {
                         LiteElement::Command(span, command) => {
+                            trace!("parsing: pipeline element: command");
                             let (expr, err) = parse_expression(
                                 working_set,
                                 &command.parts,
@@ -5309,6 +5320,7 @@ pub fn parse_block(
                             PipelineElement::Expression(*span, expr)
                         }
                         LiteElement::Redirection(span, redirection, command) => {
+                            trace!("parsing: pipeline element: redirection");
                             let (expr, err) = parse_string(
                                 working_set,
                                 command.parts[0],
@@ -6185,8 +6197,6 @@ pub fn parse(
     scoped: bool,
     expand_aliases_denylist: &[usize],
 ) -> (Block, Option<ParseError>) {
-    trace!("starting top-level parse");
-
     let mut error = None;
 
     let span_offset = working_set.next_span_start();