Pony: fix identifier and keyword

Summary:

• missing elseiftype keyword
• fix DataType of 1 or 2 characters or starting with _
• ' suffix is valid for a variable

Test Plan: Run the tests.

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: Frameworks

Tags: Framework: Syntax Highlighting, Frameworks

Differential Revision: https://phabricator.kde.org/D12569