Issue template for alpha

[samtstern]

This PR makes a few changes to issue templates:

1. Adds a separate issue template for Alpha bugs. It has a slightly different preamble and also auto-applies the alpha label.
2. Adds a new "choice" screen to direct people to the right template and also offer a link to Support.

Hard to preview in a PR but here's how the iOS SDK chooser looks:

[changeset-bot]

⚠️ No Changeset found

Latest commit: ee6ae8f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (04f2e24) and head commit (3158f11).

Test Logs

• Base (04f2e248): https://github.com/firebase/firebase-js-sdk/actions/runs/416370910
• Head (3158f111): https://github.com/firebase/firebase-js-sdk/actions/runs/421387688