Add syntax checker

[PrivateGER]

Implement a syntax checker that works. The current one is very situational and barely does anything at all.

Take a look at lib/syntax_checker.js for an idea of how to implement syntax checking functions.
You may introduce breaking changes into the project, but I would prefer it if you could keep them as smooth as possible.

This issue will likely require modification of the transpiler. It is located in lib/transpiler.js.

[klee214]

Hello! I can give a little help to enhance your program.
I will try to add some great syntax checker features!