Enable dynamically loaded postgres extensions

[var77]

• To enable dynamic linking we need to compile postgres with MAIN_MODULE=1, but it will export all postgres symbols to JS, in emscripten there's a bug related to symbol collision issue with WASI symbols. Postgres has symbol proc_exit and symbol with the same name also exists in WASI runtime, so after exporting this symbol we are getting runtime error that proc_exit from wasi_snapshot_preview_1 is not found. I have renamed postgres symbol to pg_proc_exit to fix the issue until the fix from emscripten will be available. This may only break extensions which are using the proc_exit function from postgres.
• Fixed function pointer casts issue by applying patch: https://github.com/postgres/postgres/commit/c35ba141de1fa04373671ba24c73eb0fe4862415.patch . Thus we can drop EMULATE_FUNCTION_POINTER_CASTS flag, so dlsym could work.
• Comment out extension file check in dfmgr.c to allow passing any URL for extension wasm file as now the extension control file can contain full URL to wasm file under module_pathname