feat: expose managers + some models (#2139)

* docs: add doc comments to the manager files

* feat: export manager classes

Export all manager files + the FileSystem class from models.
FileSystem is needed as an argument to many of the manager functions.

Managers + models can be import like so
`import {GitConfigManager, GitIndexManager} from "isomorphic-git/managers"`
`import {FileSystem} from "isomorphic-git/models"`

* docs: add docs for manager files

* build: output built to managers + models folders

* docs: update comments for FielSystem file

* docs: update website docs for FileSystem.js

* docs: fix file ids

* docs: update localized strings

* chore: update tsconfig

* docs: replace FileSystem with FSClient

* docs: update defaults

Author: lukecotter