Add GitHub issue template for feature requests

[oxygen-dioxide]

Though we have QQ group and discord community for feature requests, a github issue template for feature requests is useful because:

• This is is highly requested in the community.
• GitHub issue is better at searching and referencing. For example, searching for "voicing" in discord gives unrelated results containing "voice". Also GitHub issue is googleable
• Some users don't want to create a discord account just to post feature requests.