Add TypeScript typings for tree-shakable version of the library

[lawrence-forooghian]

Expose typings for the tree-shakable version of the library added in https://ably.atlassian.net/browse/SDK-3719 — that is, the BaseRest and BaseRealtime classes, as well as the tree-shakable modules such as Rest etc. And expand the testing added in #1474 to cover the tree-shakable version of the library.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3857