feat: disable convert to uppercase and lowercase in visual mode

[dgokcin]

Summary

This pull request introduces a new keymap configuration to disable the conversion to uppercase and lowercase in visual mode within the VSCode plugin configuration for Neovim.

Changes

• Added a keymap to disable u and `U` in visual mode by mapping them to ``.
• This change prevents accidental case conversion when using visual mode.

Additional Notes

• The change is encapsulated within the condition that checks if the environment is not VSCode, ensuring it does not affect other environments.
• The keymap is set with `noremap` and `silent` options to ensure no recursive mapping and to suppress any output or error messages.