Support building the interpreter on Windows

[HertzDevil]

This PR includes the final bits necessary to enable interpreter builds on Windows MSVC. Requires #12391. To set up:

• Ensure your pre-existing Crystal installation is new enough by verifying that ffi.lib exists in CRYSTAL_LIBRARY_PATH (it was added after 1.5.0, so you might have to build it yourself or grab it from the CI artifacts).
• Create the alternate CRYSTAL_LIBRARY_PATH DLL directory with gc.dll and pcre.dll according to the first 3 steps here. Do not set CRYSTAL_LIBRARY_PATH in the terminal that rebuilds the compiler.
• (Optional) Build also libiconv.dll and the iconv.lib import library:
  • Following these workflow steps, except that this time MultiThreaded becomes MultiThreadedDLL and /p:Configuration=ReleaseStatic becomes /p:Configuration=Release.
  • Copy lib64\libiconv.lib to iconv.lib inside the DLL directory.
  • Copy lib64\libiconv.dll to libiconv.dll inside the DLL directory. Do not rename the DLL - the base name is hardcoded in the import library.
• Run make -fMakefile.win interpreter=1.
• In a separate terminal, point CRYSTAL_LIBRARY_PATH to the DLL directory, and also set CRYSTAL_OPTS=-Dpreview_dll. You will have to add -Dwithout_iconv unless you did the optional steps above.
• Now bin\crystal i should start an interpreter REPL session. set CRYSTAL_INTERPRETER_LOADER_INFO=1 also works on Windows.

The main outstanding issues are tracked in #12396.