Support pinning java-version as "latest"

[Copilot]

Description:

Adds a latest alias for java-version that resolves to the newest available stable release, enabling workflows to test against bleeding-edge Java without pinning a specific major on every release. Scoped to latest only (the separate lts request is out of scope).

- uses: actions/setup-java@v5
  with:
    distribution: temurin
    java-version: latest

• Central resolution (src/distributions/base-installer.ts): normalizeVersion maps latest (case-insensitive) to the SemVer wildcard x, which flows through the existing highest-match selection. This covers all range-matching distributions (Temurin, Zulu, Adopt, Liberica, Semeru, Dragonwell, Kona, SapMachine, JetBrains, Microsoft).
• Corretto (src/distributions/corretto/installer.ts): only matches exact majors, so the wildcard is resolved to the newest available major, with a safe fallback to the standard not-found error when no numeric major exists.
• Docs (README.md): documented the latest alias under supported version syntax.
• Tests: normalizeVersion('latest') variants and Corretto latest resolution.

Note: URL-constructing distributions (Oracle, GraalVM) don't enumerate a resolvable latest major, so latest is unsupported there and surfaces a clear version-not-found error.

Check list:

• Ran npm run check locally (format, lint, build, test) and all checks pass.
• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.