Implement basics of a AST -> JS generator

Summary:
For a list of currently supported language features,
please see the commits below.

It relates to T509

[Compiler] Start

[Compiler] Add basic integration test for JS Generator

[Compiler] Make first integration test work

• assignments
• post-/predecrements
• post-/preincrements

[Compiler] Adopt integration name class to naming schema

[Compiler] Start work on functions

[Compiler] Refactor to use stack only

This allows for a more consequent concept.

• add handling of lists for SourceElements and StatementList needed to have same pass level of integration tests as before the refactoring
• change macros to reflect new concept
• introduces a new error type InternalError this error is created if the stack was not correctly reduced, which should not happen in practice

[Compiler] Finish work on functions

[Compiler] Better logic for nullptr members

• no more "" on stack
• more expressive logic
• generates code for var i correctly

[Compiler] Complete binary operations

[Compiler] Make function declarations more robust

• handle multiple parameters more correctly
• handle empty body correctly
• move integration test for EmptyStatement in its own file

[Compiler] Make functoin declaration lists work

[Compiler] Make return statements work

[Compiler] Implement branching

[Compiler] Make switch statement work

[Compiler] Make test better readable and maintainable

Reviewers: akreuzkamp

Reviewed By: akreuzkamp

Differential Revision: https://phabricator.kde.org/D350