[JS] Table getByteLength and indexOf don't work

The functions table.getByteLength() and table.indexOf() don't return the correct values.

They are bound dynamically to the Table class, in a way I don't fully understand, with the following code:

https://github.com/apache/arrow/blob/1b796ec3f9caeb5e86e3348ba940bef8d95915c5/js/src/table.ts#L378-L390

The other functions like that, get(), set(), and isValid() all seem to work.  However, getByteLength() and indexOf() return the placeholder/sentinel values of 0 and -1 respectively that are defined in the no-op code here: https://github.com/apache/arrow/blob/1b796ec3f9caeb5e86e3348ba940bef8d95915c5/js/src/table.ts#L207-L221, which I assume is to generate the right type definitions, and thus documentation.

It's fairly simple for a user to implement the right logic themselves (at least for getByteLength) and it's a quick patch to define the functions normally instead of on the prototype, e.g.:

    /**
     * Get the size in bytes of an element by index.
     * @param index The index at which to get the byteLength.
     */
    // @ts-ignore
    public getByteLength(index: number): number { return this.data[index].byteLength; }
    /**
     * Get the size in bytes of a table.
     */
    //@ts-ignore
    public getByteLength(): number { 
        return this.data.map((batch) => batch.byteLength).reduce((sum, newLength) => sum + newLength);
    } 

I'd be happy to send this as a PR if that's an OK alternative to the way it's currently implemented.

Here's a Github repo of a minimal reproduction of the issue in NodeJS:
https://github.com/alexkreidler/apache-arrow-js-small-bug
 
And an observable notebook for in the browser (although I couldn't get ESM working): https://observablehq.com/@08027ecfa2b2f7bb/arrow-7-canary
 
Thanks to all for your work on Arrow!