feat: PathRef improvements and drop deno_std fs & path re-exports

[dsherret]

PathRef:

• startsWith(path: PathRef | URL | string): boolean
• endsWith(path: PathRef | URL | string): boolean
• components(): Generator<string>
• ensureDir(): Promise<this>
• ensureDirSync(): this
• ensureFile(): Promise<this>
• ensureFileSync(): this

Removes $.fs and $.path re-exports of deno_std.

Closes #126
Closes #106

Part of #105