[file_packager.py] Add --export-es6 to file_packager.py

[lkwinta]

I have added modularize option flag to file_packager.py script, like in main js generator - related to issue#24504

I think it will be helpful since in modern world of react it is inconvinent to add generated files as <script/> tags.
This pull request introduces a new --export-es6 option to the file_packager.py tool, enabling the generation of modularized JavaScript output. It enables to import generated JS loading stub into ES6 environment. The main module which will take effect of script execution is now passed as script argument.

Use scenario:

emcc -s MODULARIZE -s EXPORT_NAME=MainModule ...
file_packager.py --export-es6 ...

import MainModule from 'main_module_wasm'

// two datafiles that will be loaded at runtime with module
import { default as loadDataFile_specialName } from 'data_file_preload' 
import loadDataFile from `data_file_preload2`

var preMod = {
   // i. e. locateFile
}

var mod = MainModule(preMod)
mod.then(async (module) =>  {
    await loadDataFile(module);
    await loadDataFile_specialName(module;

    // module has now loaded data file in VFS
});

closes: #24504

[grzanka]

@sbc100 could you please take a look ?

[sbc100]

Can you explain a little more in the PR description exactly what --modularize is doing here? It looks like its generating an ES6 module from the data files, is that right? How is the generated module supposed to interact with the program into which the files are to be loaded?

[lkwinta]

Can you explain a little more in the PR description exactly what --modularize is doing here? It looks like its generating an ES6 module from the data files, is that right? How is the generated module supposed to interact with the program into which the files are to be loaded?

I have added some use case description.

As described in the related issue we are just trying to make the dynamic loading of large necessary dependencies inside web workers run from react application. Together with XHR lazy loading this seems to be only valid solution to our problem that might solve our problem.

[lkwinta]

@sbc100 what do you think about it now?

[sbc100]

Looks like we still need a test for this.

And might be worth adding to the changelog.

Otherwise lgtm.

[lkwinta]

Looks like we still need a test for this.

And might be worth adding to the changelog.

Otherwise lgtm.

I have added two simple tests - from what I have seen file_packager.py tests aren't really extensive.