♻️ 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_module/main.cppm

@@ -7,7 +7,7 @@ import std;
 import math;
 import math2;
 
-int main()
+export int Run()
 {
     std::cout << "hello std module\n";
     std::vector<int> v{ 1, 2, 3 };