Changed the TSLint configuration to be more consistent with Theia. The most notable change is the requirement of semicolons, which were previously banned in the sprotty code base.
Added a few more TSLint rules that could be useful by forbidding error-prone JavaScript features (ever heard of the comma operator?).
Added a VS Code configuration file to enable the TSLint extension.
Applied auto-fixes of the TSLint extension to adapt the code base to the new rule set.