Add script to create amalgamated build

[bnoordhuis]

For your reviewing pleasure:

• first commit is Add std.writeFile #932

• quickjs-libc is made optional through a CONFIG_QUICKJS_LIBC define, actual name TBD

• needs hooking up to the release CI (edit: and should probably be continuously tested)

[saghul]

• quickjs-libc is made optional through a CONFIG_QUICKJS_LIBC define, actual name TBD

Following the CMake convention how about QJS_BUILD_LIBC?

• needs hooking up to the release CI (edit: and should probably be continuously tested)

Happy to help if you need me to!