feat: add language dropdown to code_block nodes

[b-kelly]

Adds the ability to choose the language for codeblocks in richtext mode. Opening as a draft because all the code is there, but it needs a design pass before being merged.

Fixes #28

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[b-kelly]

@aaronshekey One of the reasons I didn't go with the s-select originally is because it covers too much text. What should we do in cases like this:

If possible, I don't want to break the flow of the doc by placing the select outside of the code_block. Perhaps, we could hide that select if the user's cursor isn't in the node or take a similar approach as Jira:

[phoebe-leung]

In reference to the JIRA ticket and other chat threads in Slack:

@b-kelly I was thinking something like this

Much of this is taken from what Collectives is doing for their tag dropdown like we talked about in chat. For the code selector though, we’d need to have the dropdown show at the bottom of the code block so that it doesn't block the middle of where people are typing any time they click into a part of the code block.

In the screenshot above I have it default to no language selected, but it seems like maybe we should have PlainText as the default?