fix(path): normalize path slashes on Windows #37729

Problem:
On Windows, path separators may become inconsistent for various reasons,
which makes normalization quite painful.

Solution:
Normalize paths to `/` at the entry boundaries and always use it
internally, converting back only in rare cases where `\` is really
needed (e.g. cmd.exe/bat scripts?).

This is the first commit in a series of incremental steps.

Note:
* some funcs won't respect shellslash. e.g. `expand/fnamemodify`
* some funcs still respect shellslash, but will be updated in a follow
  PR. e.g. `ex_pwd/f_chdir/f_getcwd`
* uv's built-in funcs always return `\`. e.g. `uv.cwd/uv.exepath`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/vimscript/executable_spec.lua

@@ -19,13 +19,13 @@ describe('executable()', function()
   end)
 
   if is_os('win') then
-    it('exepath respects shellslash', function()
+    it('exepath returns consistent slashes #13787', function()
       -- test/ cannot be a symlink in this test.
       n.api.nvim_set_current_dir(t.paths.test_source_path)
 
       command('let $PATH = fnamemodify("./test/functional/fixtures/bin", ":p")')
       eq(
-        [[test\functional\fixtures\bin\null.CMD]],
+        [[test/functional/fixtures/bin/null.CMD]],
         call('fnamemodify', call('exepath', 'null'), ':.')
       )
       command('set shellslash')
@@ -35,12 +35,12 @@ describe('executable()', function()
       )
     end)
 
-    it('stdpath respects shellslash', function()
+    it('stdpath returns consistent slashes #13787', function()
       -- Needs to check paths relative to repo root dir.
       n.api.nvim_set_current_dir(t.paths.test_source_path)
 
       t.matches(
-        [[build\Xtest_xdg[%w_]*\share\nvim%-data]],
+        [[build/Xtest_xdg[%w_]*/share/nvim%-data]],
         call('fnamemodify', call('stdpath', 'data'), ':.')
       )
       command('set shellslash')