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

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.