Implement saveAfterFormat option

[alastairsounds]

Adds multiFormatter.saveAfterFormat setting to control whether manual format commands save the file. Fixes #30.

Default true preserves today's behavior exactly. Existing users see no change. Setting the value to false formats the document without saving. Ctrl+S with editor.formatOnSave still saves.

Changes

• New multiFormatter.saveAfterFormat setting in package.json (boolean, default true, language-overridable scope)
• New workspace.onWillSaveTextDocument listener in src/formatter.ts to detect format-on-save
• Replaced static editor.formatOnSave check with real signal about the current invocation

Behavior

Trigger	saveAfterFormat: true	saveAfterFormat: false
MultiFormat Document	saves	does not save
Format Document (⇧⌥F)	saves	does not save
Ctrl+S with editor.formatOnSave	saves	saves