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.

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")
+}