Rework insert mode (check status dynamically).
Rework insert mode (check status dynamically).

This removes @insertModeLock and instead always checks whether we are
in insert mode dynamically (as we previously did only for contentEditable).

@mrmr1993... Would you be able to check this, please?  It is intended to
be mostly a no-op.

Fixes #2841.

smblott-github/vimium,philc/vimium,decaffeinate-examples/vimium,smblott-github/vimium,decaffeinate-examples/vimium,gdh1995/vimium,smblott-github/vimium,decaffeinate-examples/vimium,poacher2k/vimium,philc/vimium,poacher2k/vimium,poacher2k/vimium,gdh1995/vimium,philc/vimium,gdh1995/vimium