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.

src/checker.js

@@ -0,0 +1,37 @@
+const childProcess = require("node:child_process")
+
+function getJavaVersionOutput(defaultCommand = "java") {
+    try {
+        const command = childProcess.spawnSync(defaultCommand, ["-version"], {
+            encoding: "utf-8",
+            windowsHide: true,
+        })
+
+        if (command.error) {
+            return null
+        }
+
+        return command.stderr || command.stdout || ""
+    } catch {
+        return null
+    }
+}
+
+function parseJavaOutput(output) {
+    if (!output || typeof output !== "string") {
+        return null
+    }
+    
+    const match = output.match(/version\s+"(?:1\.)?(\d+)/i)
+    return match ? parseInt(match[1], 10) : null
+}
+
+function isJavaCompatible(output, requiredMajor) {
+    return parseJavaOutput(output) === requiredMajor
+}
+
+module.exports = {
+    getJavaVersionOutput,
+    isJavaCompatible,
+    parseJavaOutput,
+}