feat(maven): unlink and re-add Maven projects (#297)

As in IntelliJ IDEA, a root Maven project can be unlinked from the Maven tool window's context menu (Unlink Maven Projects, with confirmation) and linked again with the toolbar's Add Maven Projects (pick a pom.xml). Unlinked root POMs are stored in the project's Maven settings (unlinkedProjects). ide-host computes the unlinked roots and their modules once with ide-build and applies the same exclusion to the build model, Maven sync, project source roots, detected run configurations and the Java language server import (java.import.exclusions). The project's files stay visible in the Project view as plain files.

Refs #271

src/features/settings/__tests__/languages.test.ts

@@ -31,7 +31,7 @@ describe("languages settings", () => {
     expect(v.java.jdtlsVmArgs).toBe("-Xmx2g");
     expect(v.java.jdk).toBe("");
     expect(v.java.jdtlsJdk).toBe("/opt/jdk-25");
-    expect(v.java.maven).toEqual({ home: "/opt/maven", userSettings: "", localRepository: "" });
+    expect(v.java.maven).toEqual({ home: "/opt/maven", userSettings: "", localRepository: "", unlinkedProjects: [] });
     expect(validateSettings({}).languages).toEqual(DEFAULT_LANGUAGES);
   });