[Next][Fix] Remove require() in index.js for scriptDir

[CharlieFRuan]

Overview

This PR post-processes web-llm's index.js by replacing all new (require('u' + 'rl').URL)('file:' + __filename).href with "MLC_DUMMY_PATH":

var _scriptDir = 
(typeof document === 'undefined' && typeof location === 'undefined' ? 
  "MLC_DUMMY_PATH": 
  typeof document === 'undefined' ? 
    location.href : 
    (document.currentScript && document.currentScript.src || new URL('index.js', document.baseURI).href)
);

which previously would raise error as shown in #383 and other issues.

Other occurrences of "MLC_DUMMY_PATH" are only in branches of if (ENVIRONMENT_IS_NODE) in runtime, which we do not consider / support as of now.

We use "MLC_DUMMY_PATH" instead of null because we do not expect the value to be used at all. If that is not the case, it would be easier to debug with "MLC_DUMMY_PATH".

Details

When building projects that use web-llm with next (e.g. examples/next-simple-chat), the compile time would complain about the call for require(); runtime does not run into it because document is not undefined when evaluating _scriptDir. Other examples, like examples/chrome-extension, do not have this issue because they build with parcel, which would fix it for us with @parcel/resolver-default:

This PR's fix does not affect correctness because, by inspecting index.js, _scriptDir is used to populate scriptDirectory, which is used in the function locateFile(), which currently is only used for wasmBinaryFile (but isDataURI(wasmBinaryFile) would never evaluate to false):

function locateFile(path) {
    if (Module["locateFile"]) { return Module["locateFile"](path, scriptDirectory) }
    return scriptDirectory + path
}

if (!isDataURI(wasmBinaryFile)) {
    wasmBinaryFile = locateFile(wasmBinaryFile);
}

We also do not remove other require() in index.js as of now, as from the current understanding, they would not cause issues -- but we can come back later when they do.

One observation that is not yet explainable is that, if we set "@mlc-ai/web-llm": "^0.2.35", in examples/next-simple-chat/package.json, #383 would be observed. However, if we use "@mlc-ai/web-llm": "../..",, no issue is observed -- we are able to use require() in compile time.