fix "formatting on save" potentially running on outdated document state

[rchl]

Issue reported on Discord where with both "code actions on save" and "formatting on save" enabled, the formatting would be done on an outdated document state if code actions have triggered an edit.

Reproducible with a go document like:

package main

	func main() {
		fmt.Println("Hello1")
	}

and following project settings:

	"settings": {
		"lsp_format_on_save": true,
		"lsp_code_actions_on_save": {
			"source.organizeImports": true,
		}
	}

That resulted in broken edit where part of the added import would be missing.

The FormattingTask did trigger _purge_changes_async() on start but since the tasks ran in one continuous flow on the async thread, we didn't get to handle ST's on_text_changed notification in between tasks.

Fix by starting next task from set_timeout_async to allow pending events to trigger.