Drop legacy targets

[ericvergnaud]

Removing legacy targets, only keeping:

• Java = reference target
• Kotlin = source for WebAssembly
• TypeScript = source for upcoming TypeScript wrapper (but not testing it anymore)

[ericvergnaud]

Fixes #2

[KvanTTT]

Please remove also these files:

• .clang-formal
• Package.swift
• *.md files in doc related to other targets:
  • ace-javascript-target.md
  • cpp-target.md
  • csharp-target.md
  • dart-target.md
  • go-changes.md
  • go-target.md
  • javascript-target.md
  • php-target.md
  • python-target.md
  • swift-target.md
  • targets.md

Also, ideally, clean-up other files with target-related things:

• .gitignore contains a lot of not actual lines line runtime/Cpp/_deps/ and so on
• .md files in doc directories:
  • creating-a-language-target.md (something about PHP)
  • IDEs.md (outdated targets)
  • interpreters.md (something about C++)
  • releasing-antlr.md
  • and some other

[ericvergnaud]

Updating the docs is a lot of work, and it's too early days to do that, I've created ticket #18