♻️ refactor(lsp_server): move entrypoints out of named modules

- export Run() from launcher module and add non-module main TU
- apply same pattern to module-based test executables
- migrate utils/string implementation fully into string.cppm and drop string.cpp
- point clangd to linux build compilation database

lsp-server/test/test_ast/main.cc

@@ -0,0 +1,6 @@
+import lsp.test.ast.main;
+
+int main(int argc, char* argv[])
+{
+    return Run(argc, argv);
+}