Simplify ResizingTextEdit::isFirstWord to avoid invalid QString index warnings

[zneix]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

As pointed out in #3234 (comment) I've localized the issue and fixed it.
Turns out that our complicated logic with iterating over QTextEdit's plainText and checking whether it contains space or not can be just replaced with a simple QString::contains which does exactly the same thing as our function did before.
While this might be a bit silly logic for detecting a first word, this is something to think of in the future, not something I'm focusing on approaching in this PR.

I've confirmed it Fixes #3234

[zneix]

Considering this warning/bug was quite annoying, noticed by quite some ppl and also breaking for Qt6 builds I've decided to put a changelog entry for it.

[Mm2PL]

looks good to me, BroBalt

[pajlada]

Initial fix didn't account for the feature where if you move your cursor to the start an already started message: <CURSOR> hello!! and type @pajl and press tab, it would include a comma since it was autocompleting as the first word.

The commit I added brings that feature back while keeping the improved (and much simpler) check

Example variable values:
Input box: Hello @pajl<CURSOR>
plainText: Hello @pajl
portionBeforeCursor: Hello @pajl
Autocompleting will insert @pajlada

Input box: Hello @pajl<CURSOR>xD
plainText: Hello @pajlxD
portionBeforeCursor: Hello @pajl
Autocompleting will insert @pajlada

Input box: @pajl<CURSOR> Hello xD
plainText: @pajl Hello xD
portionBeforeCursor: @pajl
Autocompleting will insert @pajlada,

[zneix]

Thanks so much for noticing my oversight and making a comment covering it 👍