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/executable_spec.lua

@@ -2,7 +2,7 @@ local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 
 local eq, clear, call, write_file, command = t.eq, n.clear, n.call, t.write_file, n.command
-local eval = n.eval
+local eval, fn = n.eval, n.fn
 local is_os = t.is_os
 local pcall_err = t.pcall_err
 
@@ -18,38 +18,26 @@ describe('executable()', function()
     eq(1, call('executable', 'false'))
   end)
 
-  if is_os('win') then
-    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)
+  it('exepath respects shellslash #13787', function()
+    t.skip(not is_os('win'), 'N/A for non-Windows')
+    -- 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]],
-        call('fnamemodify', call('exepath', 'null'), ':.')
-      )
-      command('set shellslash')
-      eq(
-        'test/functional/fixtures/bin/null.CMD',
-        call('fnamemodify', call('exepath', 'null'), ':.')
-      )
-    end)
+    command('let $PATH = fnamemodify("./test/functional/fixtures/bin", ":p")')
+    eq(([[%s\test\functional\fixtures\bin\null.CMD]]):format(fn.getcwd()), fn.exepath('null'))
+    command('set shellslash')
+    eq(('%s/test/functional/fixtures/bin/null.CMD'):format(fn.getcwd()), fn.exepath('null'))
+  end)
 
-    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)
+  it('stdpath returns consistent slashes #13787', function()
+    t.skip(not is_os('win'), 'N/A for non-Windows')
+    -- 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]],
-        call('fnamemodify', call('stdpath', 'data'), ':.')
-      )
-      command('set shellslash')
-      t.matches(
-        'build/Xtest_xdg[%w_]*/share/nvim%-data',
-        call('fnamemodify', call('stdpath', 'data'), ':.')
-      )
-    end)
-  end
+    t.matches('build/Xtest_xdg[%w_]*/share/nvim%-data', fn.stdpath('data'))
+    command('set shellslash')
+    t.matches('build/Xtest_xdg[%w_]*/share/nvim%-data', fn.stdpath('data'))
+  end)
 
   it('fails for invalid values', function()
     for _, input in ipairs({ 'v:null', 'v:true', 'v:false', '{}', '[]' }) do