Create helix-stdx crate for stdlib extensions

helix-stdx is meant to carry extensions to the stdlib or low-level
dependencies that are useful in all other crates. This commit starts
with all of the path functions from helix-core and the CWD tracking that
lived in helix-loader.

The CWD tracking in helix-loader was previously unable to call the
canonicalization functions in helix-core. Switching to our custom
canonicalization code should make no noticeable difference though
since `std::env::current_dir` returns a canonicalized path with
symlinks resolved (at least on unix).

helix-stdx/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "helix-stdx"
+description = "Standard library extensions"
+include = ["src/**/*", "README.md"]
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+categories.workspace = true
+repository.workspace = true
+homepage.workspace = true
+
+[dependencies]
+dunce = "1.0"
+etcetera = "0.8"
+
+[dev-dependencies]
+tempfile = "3.9"