Use helix-stdx tilde expansion and normalization for HELIX_RUNTIME paths

Previously this wasn't possible since helix-core depends on helix-loader,
so helix-loader couldn't use helix-core's path extensions.

We use the path normalization/canonicalization for the runtime directory
provided by the HELIX_RUNTIME environment variable. This improves a
scenario where you set a path containing a tilde. Now that path will be
expanded and normalized.

helix-loader/Cargo.toml

@@ -15,6 +15,8 @@ name = "hx-loader"
 path = "src/main.rs"
 
 [dependencies]
+helix-stdx = { path = "../helix-stdx" }
+
 anyhow = "1"
 serde = { version = "1.0", features = ["derive"] }
 toml = "0.7"