Make typescript an optional peer dependency

[veksa]

dependency-tree pins typescript in dependencies, which breaks dedup: anyone whose TypeScript falls outside ^6.0.3 gets an extra copy of the compiler nested inside node_modules/dependency-tree. On a project with typescript@5.9.3 that's 114 MB → 91 MB.

typescript is only needed here to pre-parse a tsConfig given as a path, and it is already required lazily, so the peer is marked optional. The require goes through a loadTypeScript() helper that reports a missing peer dependency, or one that no longer carries the config-parsing API, instead of failing on an undefined property. That matters for typescript@7, which is already the latest tag and is a rewrite without readJsonConfigFile or parseJsonSourceFileConfigFileContent. No version is inspected anywhere; the range in package.json matches what detective-typescript and detective-vue2 already declare.

One related fix in index.js: getDependencies() swallows everything precinct throws and returns an empty list, which is right for a file that fails to parse but wrong for a missing peer dependency — that silently produced an empty tree instead of an error. Errors tagged ERR_TYPESCRIPT_UNAVAILABLE are now rethrown. Unrelated parse failures behave exactly as before, and there's a test pinning that down.