feat(exmode): "1q:", :exmode #41010

Problem:
Want `gQ` for _le multicursor_.

Solution:
- Don't use `gQ` for exmode.
- Introduce `:exmode`.
- Introduce `[count]q:` as an alias to `:exmode`.

test/functional/api/vim_spec.lua

@@ -5321,7 +5321,7 @@ describe('API', function()
       insert([[
         foo
         bar]])
-      feed('gQ1')
+      feed('1q:1')
       screen:expect({ any = vim.pesc('{1::}1^') })
       eq('Parsing command-line', pcall_err(api.nvim_parse_cmd, '', {}))
       -- Executing the line still auto-prints it.