Add simpler mechanism for define new JS preprocessor functions/macros.

[sbc100]

Currently we have a fixed set of processor macros/function that are mostly defined in parseTools.js

Adding new macros in JS libraries is possible, but its rather ugly syntax, and has some limitations:

{{{
globalThis.newMacro = () = { .. }
}}}

A major drawback (IIUC) of doing this is that the macros are not available during preprocessing since macros expansion happens after preprocessing. So to use these macros in a preprocessor directive one would need define them in a separate file that is loaded first. e.g. --js-library=my_macros.js --js-library=my_library.js. Here the macros defined in my_macros.js would be available in my_library.js.

One way to improve this would be to allow my_library.js to somehow #include "my_macros.js" .. however that isn't currently how the #include directive works. Currently #include files are placed directly into the output file and don't have macros expanded, so we would need somekind of new #include mechanism.