Add `PreModeChanged` autocmd for cancelling mode transitions

[RobertCraigie]

In some cases you may want to disable specific mode transitions that glide does by default, e.g.

glide/src/glide/browser/base/content/plugins/shims.mts

Line 34 in aba1f23

void glide.excmds.execute("mode_change insert");

.

I think an elegant way to support this, would be to introduce a new autocmd that fires before the mode actually changes, and can be used to prevent a mode change from happening.

We should update all mode changes to also include a "source", so you can tell what actually triggered a mode change.

I'm not sure what exactly this API should look like, and all the edge cases therein yet.