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/parse_keywords.rs

@@ -1,3 +1,4 @@
+use log::trace;
 use nu_path::canonicalize_with;
 use nu_protocol::{
     ast::{
@@ -3130,6 +3131,97 @@ pub fn parse_source(
     )
 }
 
+pub fn parse_where_expr(
+    working_set: &mut StateWorkingSet,
+    spans: &[Span],
+    expand_aliases_denylist: &[usize],
+) -> (Expression, Option<ParseError>) {
+    trace!("parsing: where");
+
+    if !spans.is_empty() && working_set.get_span_contents(spans[0]) != b"where" {
+        return (
+            garbage(span(spans)),
+            Some(ParseError::UnknownState(
+                "internal error: Wrong call name for 'where' command".into(),
+                span(spans),
+            )),
+        );
+    }
+
+    if spans.len() < 2 {
+        return (
+            garbage(span(spans)),
+            Some(ParseError::MissingPositional(
+                "row condition".into(),
+                span(spans),
+                "where <row_condition>".into(),
+            )),
+        );
+    }
+
+    let call = match working_set.find_decl(b"where", &Type::Any) {
+        Some(decl_id) => {
+            let ParsedInternalCall {
+                call,
+                error: mut err,
+                output,
+            } = parse_internal_call(
+                working_set,
+                spans[0],
+                &spans[1..],
+                decl_id,
+                expand_aliases_denylist,
+            );
+            let decl = working_set.get_decl(decl_id);
+
+            let call_span = span(spans);
+
+            err = check_call(call_span, &decl.signature(), &call).or(err);
+            if err.is_some() || call.has_flag("help") {
+                return (
+                    Expression {
+                        expr: Expr::Call(call),
+                        span: call_span,
+                        ty: output,
+                        custom_completion: None,
+                    },
+                    err,
+                );
+            }
+
+            call
+        }
+        None => {
+            return (
+                garbage(span(spans)),
+                Some(ParseError::UnknownState(
+                    "internal error: 'where' declaration not found".into(),
+                    span(spans),
+                )),
+            )
+        }
+    };
+
+    (
+        Expression {
+            expr: Expr::Call(call),
+            span: span(spans),
+            ty: Type::Any,
+            custom_completion: None,
+        },
+        None,
+    )
+}
+
+pub fn parse_where(
+    working_set: &mut StateWorkingSet,
+    spans: &[Span],
+    expand_aliases_denylist: &[usize],
+) -> (Pipeline, Option<ParseError>) {
+    let (expression, err) = parse_where_expr(working_set, spans, expand_aliases_denylist);
+    (Pipeline::from_vec(vec![expression]), err)
+}
+
 #[cfg(feature = "plugin")]
 pub fn parse_register(
     working_set: &mut StateWorkingSet,