feat: add dynamicImportFunction option

[keithamus]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

N/A

Description

This option allows users of esm format to rename the import() function, which right now has mixed support in browsers. By allowing this function to be renamed we can have polyfills (which cannot use the reserved import keyword but could go by a different name like importModule).

Right now if we chose to use esm format, and use dynamic imports, then the output source text uses the import() function - this fails in browsers that don't support import(). By adding this option, and renaming import() to something else, we have a chance at using dynamic imports before all browsrs support them.

[lukastaegert]

Thanks, I like the feature, implementation looks also fine (though there is still an option list in the tests that needs to be adjusted I assume). Some thoughts:

• It would be nice to get a warning that this option is ignored for non-esm targets unless the format is esm
• the option should be added to the documentation (big list of options + other option lists). I assume doing the actual polyfill is not something that can be jotted down in a few lines or maybe there is a link to one or some documentation that can be added?
• the option should be added to the CLI options list (help.md)

[keithamus]

Okay @lukastaegert I believe this addresses your points. Thanks for fixing the tests 😄

[lukastaegert]

Nice one, thanks a lot, especially for the link to the polyfill!

[tomalec]

Why is it limited to es format only?

I have a use case for thins for iife.
The code is meant to be run by Node and browser.

if(typeof fetch === 'undefined'){ // in node shim JSON modules with imported require
    const createRequire = await import('module').then(m=>m.createRequire);
    const require = createRequire(import.meta.url);
    currentVersion = require('../../package.json').version;
 } else { // in browser shim with fetch
    currentVersion = await fetch('../../package.json')
        .then(response => response.json())
        .then(json => json.version);
}

On Node it runs flawlessly, in modern browsers as well, but old Edge is throwing a Syntax Error for import(.

I would like to use dynamicImportFunction: "somethingElseForEdge" to let Edge parse the code.