Fixed GFM autocomplete regex

## What does this MR do?

Their was a bug in the regex that meant it was matching from the beginning of the GFM input text. This fixes that by splitting spaces & then checking on the last match. 

It also fixes a bug where `@#` etc. would open 2 autocomplete dropdowns.

## What are the relevant issue numbers?

Closes #25119

See merge request !7826