Json tmlanguage format + fixes and additions

[FreHu]

Original request: #11

Now the changes are split into separate commits for easier review.

I also provided a test file which is always updated with relevant changes. To get the updated highlighting, I used your vscode extension and changed the config to point at my grammar instead.

[FreHu]

Will be submitted as separate PRs. (No json format for now - although it is nicer, it either adds a dependency on tooling or causes double the work)