Bare word improvements (#8066)

# Description

This does two fixes for bare words:

* It changes completions for paths to wrap a path with backticks if it
starts with a number. This helps bare words that start with numbers be
separate from unit values
* It allows bare words wrapped with backticks to be the name of a
command. Backtick values in command positions are no longer treated as
strings

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# 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.

tests/shell/pipeline/commands/internal.rs

@@ -1258,6 +1258,13 @@ mod parse {
 
         assert!(actual.err.contains("extra positional argument"),);
     }
+
+    #[test]
+    fn ensure_backticks_are_bareword_command() {
+        let actual = nu!(cwd: ".", "`8abc123`");
+
+        assert!(actual.err.contains("was not found"),);
+    }
 }
 
 mod tilde_expansion {