parser: Add cell path literal syntax (#8493)

# Description

This adds a new cell path literal syntax for use in any value position,
not just in a context where we expect a cell path.

This can be used to assign to a variable and then later use that
variable as a cell path.

Example:
```
> let cell_path = $.a.b
> {a: {b: 3}} | get $cell_path
3
```
# User-Facing Changes

This adds the syntax `$.a.b` to universally mean the cell path `a.b`,
even in a context that doesn't expect a cell path.

# 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

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# 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

@@ -1688,6 +1688,12 @@ pub(crate) fn parse_dollar_expr(
 
     if contents.starts_with(b"$\"") || contents.starts_with(b"$'") {
         parse_string_interpolation(working_set, span, expand_aliases_denylist)
+    } else if contents.starts_with(b"$.") {
+        parse_simple_cell_path(
+            working_set,
+            Span::new(span.start + 2, span.end),
+            expand_aliases_denylist,
+        )
     } else if let (expr, None) = parse_range(working_set, span, expand_aliases_denylist) {
         (expr, None)
     } else {
@@ -2129,6 +2135,33 @@ pub fn parse_cell_path(
     (tail, error)
 }
 
+pub fn parse_simple_cell_path(
+    working_set: &mut StateWorkingSet,
+    span: Span,
+    expand_aliases_denylist: &[usize],
+) -> (Expression, Option<ParseError>) {
+    let source = working_set.get_span_contents(span);
+    let mut error = None;
+
+    let (tokens, err) = lex(source, span.start, &[b'\n', b'\r'], &[b'.', b'?'], true);
+    error = error.or(err);
+
+    let tokens = tokens.into_iter().peekable();
+
+    let (cell_path, err) = parse_cell_path(working_set, tokens, false, expand_aliases_denylist);
+    error = error.or(err);
+
+    (
+        Expression {
+            expr: Expr::CellPath(CellPath { members: cell_path }),
+            span,
+            ty: Type::CellPath,
+            custom_completion: None,
+        },
+        error,
+    )
+}
+
 pub fn parse_full_cell_path(
     working_set: &mut StateWorkingSet,
     implicit_head: Option<VarId>,
@@ -4646,29 +4679,7 @@ pub fn parse_value(
                 )
             }
         }
-        SyntaxShape::CellPath => {
-            let source = working_set.get_span_contents(span);
-            let mut error = None;
-
-            let (tokens, err) = lex(source, span.start, &[b'\n', b'\r'], &[b'.', b'?'], true);
-            error = error.or(err);
-
-            let tokens = tokens.into_iter().peekable();
-
-            let (cell_path, err) =
-                parse_cell_path(working_set, tokens, false, expand_aliases_denylist);
-            error = error.or(err);
-
-            (
-                Expression {
-                    expr: Expr::CellPath(CellPath { members: cell_path }),
-                    span,
-                    ty: Type::CellPath,
-                    custom_completion: None,
-                },
-                error,
-            )
-        }
+        SyntaxShape::CellPath => parse_simple_cell_path(working_set, span, expand_aliases_denylist),
         SyntaxShape::Boolean => {
             // Redundant, though we catch bad boolean parses here
             if bytes == b"true" || bytes == b"false" {

src/tests/test_cell_path.rs

@@ -133,3 +133,8 @@ fn list_row_optional_access_succeeds() -> TestResult {
 fn do_not_delve_too_deep_in_nested_lists() -> TestResult {
     fail_test("[[{foo: bar}]].foo", "cannot find column")
 }
+
+#[test]
+fn cell_path_literals() -> TestResult {
+    run_test("let cell_path = $.a.b; {a: {b: 3}} | get $cell_path", "3")
+}