improve word completion to use highlighting to detect word boundaries

Summary:
This allows better completion e.g. for Perl or PHP

BUG: 360340

Test Plan:
Open some Perl script.

Type:

lala_test = 1

$lala = 2

Now type

$lal

You shall now get only "lala" as proposal

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23410