[Linux] Make fl_plugin_registrar_new public

[RossComputerGuy]

By making fl_plugin_registrar_new public, it allows custom embedders to have further compatibility with existing Linux plugins.

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[flutter-dashboard]

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[robert-ancell]

Can you elaborate more on how another embedder might make use of this? This object is very tied to the Linux embedder, i.e. making use of FlBinaryMessenger, FlView etc.