[Next][Fix] Remove require() in index.js for scriptDir (mlc-ai#397)

### 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"`:

```javascript
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
mlc-ai#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`:

![image](https://github.com/mlc-ai/web-llm/assets/53290280/0b9df99a-f80e-4fed-8c19-88deb8aabfbd)

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`):

```javascript
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`,
mlc-ai#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.

Author: CharlieFRuan

cleanup-index-js.sh

@@ -4,6 +4,12 @@
 sed -e s/"const{createRequire:createRequire}=await import('module');"//g -i .backup lib/index.js
 sed -e s/"const{createRequire:createRequire}=await import('module');"//g -i .backup lib/index.js.map
 
+# Replace string "new (require('u' + 'rl').URL)('file:' + __filename).href" with "MLC_DUMMY_PATH"
+# This is required for building nextJS projects -- its compile time would complain about `require()`
+# See https://github.com/mlc-ai/web-llm/issues/383 and the fixing PR's description for more.
+sed -e s/"new (require('u' + 'rl').URL)('file:' + __filename).href"/"\"MLC_DUMMY_PATH\""/g -i .backup lib/index.js
+sed -e s/"new (require('u' + 'rl').URL)('file:' + __filename).href"/"\"MLC_DUMMY_PATH\""/g -i .backup lib/index.js.map
+
 # Cleanup backup files
 rm lib/index.js.backup
 rm lib/index.js.map.backup