MONGOSH-210: Preserve shell state when collapsing

[Anemy]

https://jira.mongodb.org/browse/MONGOSH-210

This PR makes it so the shell retains its output when its collapsed and re-opened.
It also adds a hotkey Command-L/Ctrl-L (Mac/Windows) which clears the output of the shell.

We had an error in compass-shell where signatures from the shell api weren't loading correctly, this PR fixes that by moving the signature references into the completer so that they're referenced at runtime, since we build the signatures object from the decorator. https://github.com/mongodb-js/mongosh/pull/240/files#diff-6635c7669eeb4f2b6ed7cfb2857f389eL19

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Before merging:

• Manually tested with a large amount of output to ensure its not laggy on collapse/expand. It takes a half second to expand but it works fine.

[Anemy]

@aherlihy just heads up, doing a small change on completion signatures here: https://github.com/mongodb-js/mongosh/pull/240/files#diff-6635c7669eeb4f2b6ed7cfb2857f389eL19
I think Irina had done some work on pulling the autocompleter into its own package so we can remove the cli-repl dependency from browser-repl

[mcasimir]

Looks good