[dart fix] --code enable unconfigured lints

[pq]

The current dart fix --code support works by filtering diagnostics which means that it can only be used to apply fixes for lints that are enabled via analysis options. In some cases it might be useful to allow fix to enable lints that are not explicitly enumerated in options (see e.g., #47219 (comment)).

/fyi @mit-mit @bwilkerson @davidmorgan @jacob314

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(Follow-on issue from #47219 which tracks the main dart --apply --code feature.)

[bwilkerson]

Another option that was just suggested on a chat was for a dart fix --add --code mode that would both update the analysis options file to add the lint and fix the violations.