PRP: Add Scala SBT build file extractor

[Amsamms]

Description

Adds a filesystem extractor for Scala SBT build files (.sbt) that extracts Maven package dependencies.

Closes #1022

Supported SBT dependency patterns

• Single inline dependencies: libraryDependencies += "groupId" %% "artifactId" % "version"
• Seq block dependencies: libraryDependencies ++= Seq("g" %% "a" % "v", ...)
• Version variable resolution: val ver = "1.0.0" / lazy val ver = "1.0.0" references
• All cross-version operators: %, %%, %%%
• Configuration scoping: % Test suffix (ignored for extraction)

Design decisions

Based on review feedback from #1801:

• Configurable file size limits: Default 10MB max, configurable via PluginConfig.MaxFileSizeBytes. File size checked in FileRequired() before reading.
• Regex compiled once in New(): All regex patterns are compiled during initialization and stored in the Extractor struct, avoiding repeated compilation in Extract() and the MustCompile vs Compile lint conflict.
• Stats reporting: FileRequired and Extract both report stats via stats.Collector.
• Reuses javalockfile.Metadata: SBT resolves to Maven artifacts, so we reuse the Java lockfile metadata type (no proto changes needed).
• All files end with newline: Passes linelint.

Files changed

• extractor/filesystem/language/scala/sbt/sbt.go — Extractor implementation
• extractor/filesystem/language/scala/sbt/sbt_test.go — Tests for FileRequired and Extract
• extractor/filesystem/language/scala/sbt/testdata/ — Test fixtures
• extractor/filesystem/list/list.go — Registers ScalaSource extractor
• docs/supported_inventory_types.md — Documents the new extractor

Testing

All tests pass locally on Go 1.24.2:

$ go test ./extractor/filesystem/language/scala/sbt/... -count=2
ok  github.com/google/osv-scalibr/extractor/filesystem/language/scala/sbt  0.024s
$ go vet ./extractor/filesystem/language/scala/sbt/...
$ make lint-plugger  # passes

[Amsamms]

Friendly ping @erikvarga @cuixq — this Scala SBT build file extractor PR has been open for 10 days with all 13 CI checks passing. Per the PRP inventory scope (post-Apr 9), this extractor parses build.sbt files for the Scala/Maven ecosystem. Would appreciate a review when you have capacity. Happy to address any feedback. Thanks!