Add legacy SFC emission with firrtl2

[jerryz123]

Generator will emit .fir using modern chisel.firrtl, and .sfc.fir using the firrtl2.bridge features of firrtl2 SFC.

This will be used in the future to support backwards compatibility with SFC-based flows.

Merge after #1854

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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• Did you set main as the base branch?
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• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
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• (If applicable) Did you mark the PR as Please Backport?