Add proper issue template for bug reports

[farooqkz]

How do we test it without merging? Also, is it possible to provide a better interface for test cases?

[Simon-Laux]

I think the most important are the test cases, it should explain how to get them:
https://deltachat.github.io/message-parser/

Test case should be ideally input, expected output AST and actual output AST.

[Simon-Laux]

Also, is it possible to provide a better interface for test cases?

we could change the demo site, like add a copy button to the field where the AST is displayed.

[Simon-Laux]

Anyways I'm not really sure that we need an issue template. I'm glad if people report bugs, they might not if we increase the barrier of entry / time cost of filing bugs.