Create issue and pull request templates

[jrfnl]

As per the title.

Includes checking if the new GH yaml file format for this helps any.

[jrfnl]

For inspiration: ampproject/amp-wp#6543

[jrfnl]

@schlessera I've been looking into this a little and would like to propose keeping it simple with some basic templates for now while we work out what information we actually need/are commonly missing in issues.

Once that has been distilled down to a stable template, we can look into moving to the Yaml templates.

From what I can see, the Yaml templates are a lot less flexible, so don't allow much for a user wanting to add additional information, so for now, it doesn't seem worth it being that restrictive.

[jrfnl]

Note: I've not included a pull request template in the PR as, to be honest, I'm not sure what we'd want to include in that.