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/man.lua

@@ -262,7 +262,7 @@ local function get_path(name, sect)
   -- find any that match the specified name
   --- @param v string
   local namematches = vim.tbl_filter(function(v)
-    local tail = fn.fnamemodify(v, ':t')
+    local tail = vim.fs.basename(v)
     return tail:find(name, 1, true) ~= nil
   end, results) or {}
   local sectmatches = {}
@@ -364,7 +364,7 @@ end
 --- @return string name
 --- @return string sect
 local function parse_path(path)
-  local tail = fn.fnamemodify(path, ':t')
+  local tail = vim.fs.basename(path)
   if
     path:match('%.[glx]z$')
     or path:match('%.bz2$')