Fix Completions lag on long lines

[RedCMD]

Fixes #227892

create plaintext file with snippet

abc
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

duplicate the colon's so there is about 10k-100k of them
type a at the end of that line
notice that it no longer hangs VSCode
and you now receive abc Snippet suggestion

[RedCMD]

@jrieken would you mind giving this a review thanks