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.

crates/nu-parser/src/parser.rs

@@ -76,15 +76,7 @@ pub fn is_math_expression_like(
 
     let b = bytes[0];
 
-    if b == b'('
-        || b == b'{'
-        || b == b'['
-        || b == b'$'
-        || b == b'"'
-        || b == b'\''
-        || b == b'`'
-        || b == b'-'
-    {
+    if b == b'(' || b == b'{' || b == b'[' || b == b'$' || b == b'"' || b == b'\'' || b == b'-' {
         return true;
     }