feat: move editor insert logic to useSpecialContent composable

[scmmishra]

This PR makes the following changes

1. Move createState to a editorHelpers, we didn't need it inside the component anyway
2. Remove insertMentionNode, insertCannedResponse and insertVariable and replace with with a composable with abstracted insertion logic
3. Add useSpecialContent composable along with tests for it

Motivation

The editor is one of the most complicated UI components we have, however it is too big of a component to be maintained meaningfully, and is largely untested. Refactoring it piece by piece will minimise the risk as well as get it ready for Vue 3 migration

[scmmishra]

@chatwoot/frontend hey folks, need your review on this. PTAL and let me know if there are any avenues for improvement, the plan is to rewrite this component completely in 4 to 5 iterations and ensure good tests

[scmmishra]

@pranavrajs PTAL