Add workflow to automatically assign milestone

This is a follow-up to:

• https://github.com/WordPress/performance/issues/2484

We have labels to identify which plugin(s) an issue or pull request is associated with. For example, https://github.com/WordPress/performance/pull/2469 has:

Additionally, we have the milestone:

(In reality, a PR should be able to be associated with multiple milestones because we have a monorepo. This isn't possible, however.)

As of https://github.com/WordPress/performance/pull/2513 the labels are now automatically applied.

The plugin-specific milestone can also automatically be assigned based on which plugin has the most changed lines in the PR. This milestone could then change automatically as additional changes are pushed.

As a bonus, when the changelog is being gathered for a given plugin, in addition to looking at the PRs in a plugin's specific milestone, we should also see if there are any merged PRs with the plugin's label which are associated with an open milestone. These can automatically be included in the changelog as well. This solves a manual issue of needing to make sure that a single PR with changes for multiple plugins get a changelog entry in each.

Start by reading follow-up issue #2484 and PR #2513, then inspect the existing automation that applies plugin labels. Determine how milestone selection by changed-line counts and changelog inclusion for labeled merged PRs should work; done means the workflow updates milestones as PRs change and the changelog includes qualifying cross-plugin PRs.