Add clangd target in duckdb_extension.Makefile

[freddie-freeloader]

I am using clangd in my editor, and it's very useful for navigating the duckdb-codebase.

In the main repository (duckdb) there is a clangd target in the Makefile (link to the Makefile).

It would be super useful to have such a target definition in this project, too.

---

What I am using right now, is creating a debug build in build/debug and adding a .clangd-file with

CompileFlags:
  CompilationDatabase: build/debug
  Add: -Wno-unqualified-std-cast-call

This seems to work for the files in src/, but sadly not for all source files in duckdb/.

---

Unfortunately, I don't understand CMake and clangd good enough, to add the proper commands to the duckdb_extension.Makefile — so some help here would be very appreciated. ☺️