Support pre-release publishing with SNAPSHOT language server resolution

[raleigh-g-thompson]

Add support for publishing pre-release extension builds that track -SNAPSHOT language server artifacts.

• javaServiceInstaller.ts: resolve SNAPSHOT JARs from Sonatype Central snapshots repo via maven-metadata.xml (timestamped artifact lookup); always re-download SNAPSHOT versions (no caching)
• package.json: strip -SNAPSHOT from extension version (vsce requires valid semver); add publish and publish:prerelease npm scripts
• README.md: replace odd/even minor versioning scheme with vsce publish --pre-release docs and updated release process