Avoid spellchecking the `f` in an `f-string`

[m-zat]

Before this commit the spellchecker highlights everything in the following:

f'correct word' ^^^^^^^^^

f'wierd word' ^^^^^^^

By adding the matchgroup directive, the start and end of the region are not considered when spellchecking. The examples become:

f'correct word'

f'weird word' ^^^^^