set up json5 completion demo and handle edge cases

[acao]

this adds working completion in a lot of cases, but there are some broken edge cases

here are some examples of the working completions across json4 and json5, both improvements and existing completion-examples.webm

Todo:

• tests! (I learned I need to call state.focus() before state.setCursor() to mock autocomplete!)

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[acao]

closing because #36 is the real way to do this, and I will move the hover fixes into their own PR