[SDK-3717] Add tree-shakable BaseClient and Rest exports

[owenpearson]

Resolves #1374

Refactors the existing Rest class into BaseClient and an encapsulated Rest class which can be passed in to the BaseClient constructor to allow access to the rest interface.

The tree-shakable modules are currently available as an esm module from 'ably/modules', example usage:

import { BaseClient, Rest } from 'ably/modules';

const client = new BaseClient(options, { Rest });

A new npm script (npm run modulereport) is added which displays information about the size of each module in the bundle-report workflow. Currently (76d2ab0) the output shows:

BaseClient: 61.63kb
BaseClient + Rest: 78.56kb

These numbers will be lower after we encapsulate further functionality into tree-shakable modules.

[lawrence-forooghian]

I'm approving (with one comment), but as discussed offline, I'm still not sure whether it makes sense to combine the realtime and REST functionality into a single BaseClient class. My preference would be to resolve this matter before merging this PR, but if you'd prefer to defer the decision until a bit later then that's cool too 👍

[jamienewcomb]

@owenpearson can we remember to link PRs to the jira issue they are fixing please. Happy to have a discussion about why this is required need be.

It can be done by tagging the issue key in the title. see ably/ably-go#596
Or by adding it to the PR description. see ably/ably-java#966

This ask is now reflected in: https://github.com/ably/engineering/blob/main/best-practices/pull-requests.md

[lawrence-forooghian]

Can we update the format task to include this file?

[lawrence-forooghian]

Replaced by #1417.