Support per-execution scalac configuration (#29)

[jozanek]

Summary

Fixes #29.

Maven lets the scala-maven-plugin <configuration> be overridden on a per-execution basis, so the compile and testCompile goals can carry different scalac options — e.g. enabling kind-projector for one but not the other.

Previously the plugin merged all executions into a single union config and applied it to both the compile and test Bloop projects, so per-goal differences were lost: Maven would compile the project fine while Bloop/Metals diverged.

What changed

initializeMojo now derives each goal's scalac options from the config Maven itself would pass to scala:compile / scala:testCompile: the plugin-level <configuration> merged with the <configuration> of the executions bound to that goal (execution-level dominating). The resulting compile/test arg lists are stashed on BloopMojo and selected per configuration when writing each Bloop project.

Notable details:

• DOM cloning — Xpp3Dom.mergeXpp3Dom mutates its dominant argument, so every DOM is deep-cloned before merging. Without this, building one view (the union) would contaminate the shared Maven-model objects reused to build the compile/test splits.
• Exact goal matching — only executions bound to compile / testCompile feed the respective split. Executions bound to unrelated goals (e.g. add-source) or no goals are excluded from the splits, but still feed the union/primary mojo, which backs Scala-version detection.
• Effective-config fallback — a goal with no per-execution override falls back to the effective default config, so scala-maven-plugin still synthesizes its defaults (-target/-release from maven.compiler.*) and honors user properties, rather than collapsing to no options.
• Resilience — getScalacArgs failures are swallowed (logged at debug) so a Java-only module that merely inherits scala-maven-plugin still gets a config written, matching the previous forgiving behavior.

Scope: only scalac options are split per goal. Scala version/context, compiler jars, organization and compile setup still come from the union view, as these don't vary per goal in practice.