fix(path): normalize separators (slashes) #39155

Problem:
Continue separators normalization, and try to keep it at the
nvim <-> external boundary, e.g., `fn.xxx`, `api.xxx`, `:xxx`

Solution:
some key changes
- normalize `$HOME-windows`
- normalize entry points of changing directory
  - `nvim_set_current_dir()`, `chdir()`
- normalize the named pipe
  - `--listen` arg, `--server` arg, `:restart`
  - `serverstart()`, `sockconnect()`, `serverstop()`
- make `expand()` respect 'shellslash' again
- clean up `did_set_shellslash`
- replace `forward_slash` with `TO_SLASH`
- remove obsolete `TMP_PATHSEPSTR`
- remove `slash_adjust` in `do_autocmd_dirchanged`?
- make `fnamemodify()` always return `/` (except when substituting
  separators via `:s`, `:gs` ?)

Note:
- these funcs still apply `slash_adjust` on return, as before:
  `:pwd`, `chdir()`, `exepath()`, `getcwd()`
- these funcs alwarys return `/`, unlike before the normalization PRs
  - `getcompletion()`, `finddir()`, `findfile()`

Also
- clean up code, comments, formatting and tests
- move `TO_SLASH` from `f_chdir` to `changedir_func`
- move `TO_SLASH` from `f_bufadd` to `buflist_new`

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

test/functional/vimscript/ctx_functions_spec.lua

@@ -322,7 +322,7 @@ describe('context functions', function()
         filter(map(add(
         getjumplist()[0], { 'bufnr': bufnr('%'), 'lnum': getcurpos()[1] }),
         'filter(
-        { "f": expand("#".v:val.bufnr.":p"), "l": v:val.lnum },
+        { "f": expand("#".v:val.bufnr.":p:gs?\\?/?"), "l": v:val.lnum },
         { k, v -> k != "l" || v != 1 })'), '!empty(v:val.f)')
         ]]):gsub('\n', ''))),
       }