Build Synlig as standalone binary

[kamilrakoczy]

Synlig is no longer a yosys plugin, it is a standalone tool now, it uses yosys as a framework for synthesis.

It is still possible to build synlig as a plugin, but it might be removed in the future.

Requires: #2539

[github-actions]

Logs difference between main branch:
https://github.com/chipsalliance/synlig-logs/compare/81b61d202de48413d5f5b53c24b962001330d228..64605-standalone-binary

[alaindargelas]

@kamilrakoczy, Please do not remove the CMake-based plugin build system ever as we need a way to build synlig as a plugin.
Being completely hard wired to a particular version of Yosys in not a good thing when one as a Yosys fork as we do.
The Makefile plugin build may go away.

[kamilrakoczy]

This PR changes the default build configuration for synlig, it does not remove the CMake-based build system.

We encourage external users to use synlig as a standalone binary, as it makes sure that synlig will work correctly without any requirements for a specific yosys version.

[alaindargelas]

Sure, that works for regular end users who want the default latest Yosys, it does not necessarily work for the Yosys-mingling ones