Add issue templates like many other repos do

[mhucka]

New issues in this repository currently have no issue templates. Users who create new issues are faced with the default "blank issue" option:

IMHO it would make for a nicer user experience if we added the issue templates we've started using on our other Quantumlib repos, as well as other Google projects like Gemini-CLI. See, for example, qsim's:

[mhucka]

Implemented in #100