Programming language class cleanup

What does this MR do and why?

Only minor changes in ProgrammingLanguage class:

• removes unused from_language method and unused/redundant :single definition for VBScript
• adds separate mapping of "single line comments" for languages, so specs now test comments independently on LANGUAGE_COMMENT_FORMATS which has two benefits: a) we don't need redundant single/single_regexp definitions in LANGUAGE_COMMENT_FORMATS and b) we can test all real possibilities for singe line comments (for VBScript we relied on :single definition while in reality single_regexp is used)
• fixes VBScript regexp issue

Related to #426815 (closed)

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