lsp: Use Path instead of String for path handling

[JunkuiZhang]

During my work on PR #22616, while trying to fix the test_reporting_fs_changes_to_language_servers test case, I noticed that we are currently handling paths using String in some places. However, this approach causes issues on Windows.

This draft PR modifies rebuild_watched_paths_inner and glob_literal_prefix. For example, take the glob_literal_prefix function modified in this PR:

assert_eq!(
    glob_literal_prefix("node_modules/**/*.js"), 
    "node_modules"
);    // This works on Unix, fails on Windows

assert_eq!(
    glob_literal_prefix("node_modules\\**\\*.js"), 
    "node_modules"
);    // This works on Windows

assert_eq!(
    glob_literal_prefix("node_modules\\**/*.js"), 
    "node_modules"
);    // This fails on Windows

The current implementation treats path as String and relies on \ as the path separator on Windows, but on Windows, both / and \ can be used as separators. This means that node_modules\**/*.js is also a valid path representation.

There are two potential solutions to this issue:

1. Continue handling paths with String, and on Windows, replace all / with \.
2. Use Path for path handling, which is the solution implemented in this PR.

Advantages of Solution 1:

• Simple and direct.

Advantages of Solution 2:

• More robust, especially in handling strip_prefix.

Currently, the logic for removing a path prefix looks like this:

let path = "/some/path/to/file.rs";
let parent = "/some/path/to";
// remove prefix
let file = path.strip_prefix(parent).unwrap();    // which is `/file.rs`
let file = file.strip_prefix("/").unwrap();

However, using Path simplifies this process and makes it more robust:

let path = Path::new("C:/path/to/src/main.rs");
let parent = Path::new("C:/path/to/src"); 
let file = path.strip_prefix(&parent).unwrap(); // which is `main.rs`

let path = Path::new("C:\\path\\to/src/main.rs");
let parent = Path::new("C:/path/to\\src\\"); 
let file = path.strip_prefix(&parent).unwrap(); // which is `main.rs`

Release Notes:

• N/A

[JunkuiZhang]

BTW, I dont know how to handle this on Windows.