jsapi-util: Move strip_unix_shebang doc comment to .cpp file
Move GjsConstString to jsapi-util.h
Remove most uses of gjs_runtime_get_context

Most of these are unnecessary, and we can't support a 1:1 runtime/context
mapping if we have more than one context.

The one leftover use of gjs_runtime_get_context is in object.cpp and it
requires a patch to gobject to fix properly.