Represent relative paths using a dedicated, separator-agnostic type (#38744)

Closes https://github.com/zed-industries/zed/issues/38690
Closes #37353

### Background

On Windows, paths are normally separated by `\`, unlike mac and linux
where they are separated by `/`. When editing code in a project that
uses a different path style than your local system (e.g. remoting from
Windows to Linux, using WSL, and collaboration between windows and unix
users), the correct separator for a path may differ from the "native"
separator.

Previously, to work around this, Zed converted paths' separators in
numerous places. This was applied to both absolute and relative paths,
leading to incorrect conversions in some cases.

### Solution

Many code paths in Zed use paths that are *relative* to either a
worktree root or a git repository. This PR introduces a dedicated type
for these paths called `RelPath`, which stores the path in the same way
regardless of host platform, and offers `Path`-like manipulation APIs.
RelPath supports *displaying* the path using either separator, so that
we can display paths in a style that is determined at runtime based on
the current project.

The representation of absolute paths is left untouched, for now.
Absolute paths are different from relative paths because (except in
contexts where we know that the path refers to the local filesystem)
they should generally be treated as opaque strings. Currently we use a
mix of types for these paths (std::path::Path, String, SanitizedPath).

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Peter Tripp <petertripp@gmail.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>
Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

crates/languages/src/python.rs

@@ -24,6 +24,7 @@ use smol::lock::OnceCell;
 use std::cmp::Ordering;
 use std::env::consts;
 use util::fs::{make_file_executable, remove_matching};
+use util::rel_path::RelPath;
 
 use parking_lot::Mutex;
 use std::str::FromStr;
@@ -52,9 +53,9 @@ impl ManifestProvider for PyprojectTomlManifestProvider {
             depth,
             delegate,
         }: ManifestQuery,
-    ) -> Option<Arc<Path>> {
+    ) -> Option<Arc<RelPath>> {
         for path in path.ancestors().take(depth) {
-            let p = path.join("pyproject.toml");
+            let p = path.join(RelPath::new("pyproject.toml").unwrap());
             if delegate.exists(&p, Some(false)) {
                 return Some(path.into());
             }
@@ -679,7 +680,7 @@ impl ContextProvider for PythonContextProvider {
                     .as_ref()
                     .and_then(|f| f.path().parent())
                     .map(Arc::from)
-                    .unwrap_or_else(|| Arc::from("".as_ref()));
+                    .unwrap_or_else(|| RelPath::empty().into());
 
                 toolchains
                     .active_toolchain(worktree_id, file_path, "Python".into(), cx)
@@ -1012,7 +1013,7 @@ impl ToolchainLister for PythonToolchainProvider {
     async fn list(
         &self,
         worktree_root: PathBuf,
-        subroot_relative_path: Arc<Path>,
+        subroot_relative_path: Arc<RelPath>,
         project_env: Option<HashMap<String, String>>,
     ) -> ToolchainList {
         let env = project_env.unwrap_or_default();
@@ -1024,7 +1025,6 @@ impl ToolchainLister for PythonToolchainProvider {
         );
         let mut config = Configuration::default();
 
-        debug_assert!(subroot_relative_path.is_relative());
         // `.ancestors()` will yield at least one path, so in case of empty `subroot_relative_path`, we'll just use
         // worktree root as the workspace directory.
         config.workspace_directories = Some(