refactor(lua): use vim.fs instead of fnamemodify

Although powerful -- especially with chained modifiers --, the
readability (and therefore maintainability) of `fnamemodify()` and its
modifiers is often worse than a function name, giving less context and
having to rely on `:h filename-modifiers`. However, it is used plenty in
the Lua stdlib:

- 16x for the basename: `fnamemodify(path, ':t')`
- 7x for the parents: `fnamemodify(path, ':h')`
- 7x for the stem (filename w/o extension): `fnamemodify(path, ':r')`
- 6x for the absolute path: `fnamemodify(path, ':p')`
- 2x for the suffix: `fnamemodify(path, ':e')`
- 2x relative to the home directory: `fnamemodify(path, ':~')`
- 1x relative to the cwd: `fnamemodify(path, ':.')`

The `fs` module in the stdlib provides a cleaner interface for most of
these path operations: `vim.fs.basename` instead of `':t'`,
`vim.fs.dirname` instead of `':h'`, `vim.fs.abspath` instead of `':p'`.
This commit refactors the runtime to use these instead of fnamemodify.

Not all fnamemodify calls are removed; some have intrinsic differences
in behavior with the `vim.fs` replacement or do not yet have a
replacement in the Lua module, i.e. `:~`, `:.`, `:e` and `:r`.

runtime/lua/vim/loader.lua

@@ -429,7 +429,7 @@ function M.enable(enable)
   M.enabled = enable
 
   if enable then
-    vim.fn.mkdir(vim.fn.fnamemodify(M.path, ':p'), 'p')
+    vim.fn.mkdir(vim.fs.abspath(M.path), 'p')
     _G.loadfile = loadfile_cached
     -- add Lua loader
     table.insert(loaders, 2, loader_cached)