feat: Implement fish-style syntax highlighting for invalid commands

Adds real-time validation of commands as they are typed. Unrecognized commands will default to red. The validation is performed efficiently by avoiding auto-discovery scans and is safely offloaded to a background task so it doesn't block the UI thread during typing. The cache is automatically cleared on AcceptLine to ensure new commands are immediately recognized.

Fix `ViFindBrace` to search for brace the same as `vim` when current …

…char is not a brace (PowerShell#4862)

Fix `ViFindBrace` to search for brace the same as `vim` when current …

…char is not a brace (PowerShell#4862)