feat: build errors indicate if they are compiler errors

[matt2e]

This used to be done with a special error struct, but that breaks as we move to language plugins communicating over gRPC.
Changes:

• Build errors now have a type (compiler or FTL)
• Position is now optional (to discern between actual positions and 0:0)
• Go compiles without using RunBuffered() as that logs errors directly. This caused LSP to pop up alerts as it didn't know if was a compiler error.