Add initial minecraft-java-manager scaffold

Initial project scaffold for minecraft-java-manager. Adds package.json and package-lock.json, README update, index.js entrypoint and test.js. Introduces src modules: checker (detects Java version), installer (downloads JRE components with redirects, SHA1 validation, progress/events and concurrency), web (fetches launcher/runtime manifests and component data), systeminfo (platform key and install path resolution) and config (endpoints). Declares dependencies adm-zip and node-downloader-helper.

index.js

@@ -0,0 +1,24 @@
+const installer = require("./src/installer")
+const checker = require("./src/checker")
+const sysinfo = require("./src/systeminfo")
+const web = require("./src/web")
+
+async function isJavaInstalledAndCompatibleFor(mcVersion, customExec = "java") {
+    const mcVersionManifestUrl = await web.getVersionManifest(mcVersion)
+    const requiredJRE = await web.getRequiredJREFor(mcVersionManifestUrl)
+    const checkJRECommand = checker.getJavaVersionOutput(customExec)
+    const isJavaCompatible = checker.isJavaCompatible(checkJRECommand, requiredJRE.majorVersion)
+    return isJavaCompatible
+}
+
+async function installJava(component, majorVersion, customInstallPath = null) {
+    const installationPath = await sysinfo.determineInstallationPath(component, majorVersion, customInstallPath)
+    const requiredComponentDetails = await web.getJREComponentDetails(component, sysinfo.getPlatformKey())
+    const requiredComponentManifest = await web.getJREComponentManifest(requiredComponentDetails[0].manifest.url)
+    return await installer.downloadJavaComponent(requiredComponentManifest, installationPath)
+}
+
+module.exports = {
+    isJavaInstalledAndCompatibleFor,
+    installJava
+}