[ssr-client] Move experimental hydrate modules to @lit-labs/ssr-client

[augustjk]

Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

SSR (@lit-labs/ssr)

Description

lit-html/experimental-hydrate.js and lit-element/experimental-hydrate-support.js should be moved to @lit-labs/ssr-client so that changes that affect only SSR & hydration concerns don't need to bump versions of the core lit-html and lit-element packages, especially when those could be considered breaking.

Recent examples:
#3667
#3677

When the modules are moved to @lit-labs/client, I'm thinking "experimental" can be dropped by virtue of it being in the @lit-labs namespace. Maybe..
lit-html/experimental-hydrate.js -> @lit-labs/ssr-client/hydrate-lit-html.js
lit-element/experimental-hydrate-support.js -> @lit-labs/ssr-client/lit-element-hydrate-support.js

lit-html/experimental-hydrate.js has code that depends on having a Node build so that will need to be added for @lit-labs/ssr-client.

We can keep the entrypoints in lit-html and lit-element (re-exported from lit) but mark as deprecated, to eventually remove. Update all docs to point to their new location instead.

Alternatives and Workarounds

N/A

[augustjk]

Type checking fails when trying to pull out lit-html/experimental-hydrate.js outside of lit-html due to /** @internal */ marked properties/methods.

from TemplateInstance:

• _parts
• _$endNode
• _$template

from ChildPart:

• _$getTemplate()