Preserve view specification in session storage

[flekschas]

I noticed when reloading the editor, the previous view specification is gone. It would be great to preserve the current view specification in session storage, so when I (as a user) reload the page, I will load the view specification I've previously worked on.

The nice thing about sessionStorage is that it's not preserved between tabs. So if I wanted to work on two view specifications at the same time I can do this without having to worry about weird caching issues.

[sehilyi]

Partly supported in #722. We can additionally support language and store two specs separately, i.e., JSON and JS codes. See below:

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I am wondering will it make sense to store language jscode and code rather than just the spec. Currently, the gosling spec is saved in session storage, which might lead to the following issues:

• the comments in the JSON editor will be removed
• the structured jscode will be changed to var spec={code} even though the users only edit in the JSON editor
• even though users edit in the js editor, they will be directed to the JSON editor when refreshing

Originally posted by @wangqianwen0418 in https://github.com/gosling-lang/gosling.js/pull/722#pullrequestreview-1000237500