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/nvim/spellfile.lua

@@ -119,6 +119,7 @@ local function ensure_target_dir()
     return dirs[1]
   end
 
+  -- vim.fs.relpath does not prepend '~/' while fnamemodify does
   dir = vim.fn.fnamemodify(dir, ':~')
   error(('cannot find a writable spell/ dir in runtimepath, and %s is not usable'):format(dir))
 end