Support cross-compiling FIRRTL source files with Scala3

[adkian-sifive]

Part of the larger work of preparing for Scala3 support

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Internal or build-related (includes code refactoring/cleanup)

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Refactor FIRRTL sources to support cross-compilation with Scala3

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: adkian-sifive / name: Aditya Naik (5bbb2e1, 6c07211, 3dd9375, 5fe182c, 8babd25, 9320b5c)
• ✅ login: jackkoenig / name: Jack Koenig (f02928d)

[jackkoenig]

I'm happy with this as is, but I think we should also add -Xsource:3 to the scalacOptions for firrtl so that we can enforce we don't regress on some of these changes. Do you want to do that here or in the next PR?

[adkian-sifive]

I like the idea of adding -Xsource:3 on a subproject basis

[jackkoenig]

Generally LGTM but see suggestion