LaTeX: fix backslash delimiter in some keywords

Summary:
BUG: 413493

FIXED-IN: 5.64.0

A delimiter before keywords of kind \command isn't required. Therefore, a DetectChar rule with lookAhead is used to detect \ at the beginning of the keywords, and thus avoid the delimiter in the keyword.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann, jpoelen

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D25139