[D] Highlight predefined versions properly

[ichordev]

Presently, only version(unittest) and version(assert) have unique highlighting. This gives the misleading impression that there's something unique about them other than them just being keywords.
Changes:

• All predefined versions are now highlighted uniformly.
• Non-predefined version identifiers just display as normal identifiers.
• Marked the version(<integer>) syntax as invalid.deprecated, as it has not been supported for over a year now, and nobody ever used it anyway...