Feat - Add expression-set-fallback slash command (#5551)

* Feat - Add expression-set-fallback slash command

* Fix ESLint

* Fix - Use proper name convention for slash command method

* Fix - Change misleading expression-set-fallback help string label

* Fix - Use normal description for expression label argument

* Update - Make expression-set-fallback case unsensitive
Also fixed array creation by removing the spread syntax and added warning.