Engine: fix check for word boundaries in the parser

[mausimus]

Parser should exclusively use is_valid_word_char to check for word boundaries (affects dash and apostrophe containing words).

[ivan-mogilko]

One request, if you make more PRs: please add some reference to the text parser, or whatever part of the program you are fixing, in commit's description. For example: "Text parser: consistently check for word boundaries" or similar.

This makes it easier for me to write changelogs and cherry pick fixes to other branches (when that's necessary).