Add language support for GYB

[fpseverino]

Add syntax highlighting and language configuration for Swift GYB (.swift.gyb) files.

Motivation:

Resolves #1455

Language support for GYB can't be implemented via sourcekit-lsp, as GYB can contain arbitrary Python.

The TextMate grammar can't be an injection grammar because GYB files have a different file extension from normal Swift files, and also because sourcekit-lsp would activate and throw a lot of errors.

Additions:

• New swift-gyb language in package.json
• Language configuration for Swift GYB with code folding, brackets definition and autoclosing
• TextMate grammar for Swift GYB
• Icons for .swift.gyb files

Known issues:

• GYB code blocks (%{...}%) and control statements (% keyword: ... % end) are not always highlighted when they are inside Swift bodies (structs, extensions and funcs, in other words when not in the top level of the file)
• Swift keywords are not always correctly highlighted when the preceding structure, function or variable name starts with a GYB block