fix(suggestion): Trigger suggestion on InsertEnter

[jsongerber]

This pull request make that the suggestion get triggered when entering insert mode.
It was not working because both InsertEnter and CursorMovedI would fire at the same time and the timer variable would be messed up in the debounce function like this:

• InsertEnter is called, copilot.timer is set to timer id 1
• CursorMovedI is called, copilot.timer is set to timer id 2
• Debounce function 1 is called, timer is 1, copilot.timer is 2, early return, copilot.timer is set to nil
• Debounce function 2 is called, timer is 2, copilot.timer is nil, early return

This pull requests fixes this, I'm testing this since yesterday and it's working fine, but I don't know the codebase very well so I'm hoping I'm not breaking something.

Also the debounce function doesn't seem to be working (with or without this pull request), it works only as a timeout, does this need to be fixed or is this the intended behavior?