Double-check default imports of glsp-client and define public and SPI API

[tortmayr]

Currently there are some inconsistencies in the default exports of the @eclipse-glsp/client package. There is no clear pattern to derive which parts of the API are considered public and it looks like modules have just been added to the public index randomly over the time. e.g. Not all default modules are added to the index which makes it hard to unload certain modules from the set of default modules. (c.f. https://github.com/eclipse-glsp/glsp/discussions/504)