fix(formatter): remove defaultFormatter dependency for codeActionsOnSave

[alphatr]

Summary

Previously, using oxfmt via editor.codeActionsOnSave required setting editor.defaultFormatter to "oxc.oxc-vscode", because the code action relied on VS Code's built-in editor.action.formatDocument command. This change removes that dependency.

What changed

• Custom format command: Registered oxc.formatDocument that directly calls the LSP textDocument/formatting request via the language client, bypassing the need for editor.defaultFormatter
• Removed defaultFormatter check: The code actions provider no longer checks whether editor.defaultFormatter is set to "oxc.oxc-vscode"
• Updated README: Clarified that editor.defaultFormatter is now optional when using codeActionsOnSave, and is only recommended for manual formatting

Test plan

• source.format.oxc in codeActionsOnSave works without setting editor.defaultFormatter
• editor.formatOnSave with editor.defaultFormatter set to "oxc.oxc-vscode" still works
• Manual formatting via Format Document command still works

[Sysix]

I do not like adding a second command specific for oxfmt, when the editor does already have a command for formatting.
Do you have any use cases where the defaultFormatter-check needs to be removed?