feat: Add const typing for Language Names

[inferrinizzard]

Description

Moves displayName, names, extension to external consts in each TargetLanguage file, and then passed as a generic arg to the base TargetLanguage class. This allows them to be statically analysed by the TypeScript compiler - leading to stricter typings for languages.

• TypeScriptFlowBaseTargetLanguage has been removed
• TypeScriptTargetLanguage and FlowTargetLanguage no longer extend from JavaScriptTargetLanguage
• all TargetLanguage classes now inherit directly from TargetLanguage instead*

*this is a minor breaking change for anyone who was inheriting from an existing TargetLanguage class, guide on how to properly extend custom classes to come

Motivation and Context

Previously, the lang option would allow for string | TargetLanguage - which would allow for any arbitrary string to be passed and would cause errors at runtime instead of at write time. This change shifts that error left and prevents any incorrect strings from being passed in the first place.

Previous Behaviour / Output

options.lang used to be:

New Behaviour / Output

now looks like:

How Has This Been Tested?

Screenshots (if appropriate):