Add support for ffigen upgrades

[aran]

Changes

Fixes #1429

This softens the ffigen requirement. ensure_tool_available still ensures ffigen is available, but doesn't cap the version.

ffigen 11 introduces a breaking change (https://pub.dev/packages/ffigen/changelog#1100) which can be disabled with a new ignore-source-errors flag. Will see from CI if that flag is needed for frb. I saw unrelated errors from ./frb_internal on master.

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.15%. Comparing base (9fdcce2) to head (0f07e1e).

❗ Current head 0f07e1e differs from pull request most recent head 2448a4d. Consider uploading reports for the commit 2448a4d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1757      +/-   ##
==========================================
- Coverage   99.26%   96.15%   -3.12%     
==========================================
  Files         358      357       -1     
  Lines       14948    14759     -189     
==========================================
- Hits        14838    14191     -647     
- Misses        110      568     +458     

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[fzyzcjy]

Good job! Only some nits