Added TSLint

[t-sauer]

I started with the recommended preset and just changed rules to match the current style or which would lead to way too complex changes (e.g. max-classes-per-file).
If you want any other rules I would be glad to change them.

[t-sauer]

I disabled the prefer-const rule.

[dgeb]

@t-sauer Thanks for your work here.

We have a few things to discuss and decide upon as a team:

• member-access - I recommend "no-public", which forbids public accessibility to be specified, because this is the default. (This is the opposite approach to that taken in this PR, which I find a bit noisy).

• Our browser support matrix for Glimmer.js, since that will drive usage of symbols, which will drive support for language features like for ... of.

We'll try to get back to you soon to minimize churn.

[t-sauer]

Seems like TSLint was already added so this PR isn't needed anymore.