This makes it possible to run the worksheet without saving the document
or manually running the dotty.worksheet.run command. The new UI is also less
distracting since you don't get a notification pop-up every time the
worksheet is being run.

This is unreliable because we change the document by adding newlines
when we receive a multi-line result for a given line of the worksheet

This way we can be sure that our output is readable in all themes.

A worksheet in an infinite loop shouldn't block the whole language
server from responding to queries. We just need to be careful to lock
the language server when operating on trees since these operations may
not be thread-safe (for example, when a symbol gets forced). We could
also allow running multiple worksheets at the same time but this commit
doesn't implement that.