Use a task queue to ensure code blocks are executed sequentially

[DavisVaughan]

Closes #509

Joint work with @lionel-

This ensures that all of the blocks are sent over in a single sequential batch, with no way for anyone else to execute their own block between two of ours, which would cause ordering issues

[DavisVaughan]

While I have not tested it, I believe this problem also exists for the Jupyter executor, as it also awaits in a loop, giving someone else the opportunity to inject a block between two executions - so you may want to file an issue for that if you agree

quarto/apps/vscode/src/host/executors.ts

Lines 63 to 78 in 06678d5

	execute: async (blocks: string[]) => {
	// if there is a cell magic then we need to execute cell-by-cell
	const hasMagic = blocks.find((block) => !!block.match(/^\s*%%\w+\s/));
	if (hasMagic) {
	for (const block of blocks) {
	await commands.executeCommand(
	"jupyter.execSelectionInteractive",
	block
	);
	}
	} else {
	const code = blocks.join("\n");
	await commands.executeCommand("jupyter.execSelectionInteractive", code);
	}
	},
	});

[lionel-]

Since this is tricky to get right, it seems like vsc-quarto should be in charge of executing the commands so that it can make sure these are run "atomically", i.e. all commands are run before yielding back to the event loop. To this end there could be a new API where language extensions provide an executeCode() callback to execute a block that vsc-quarto would then call in a loop structured similarly to the one we implemented here.

[cscheid]

(just letting you know that this change looks a lot safer now, from what I can tell!)

[DavisVaughan]

@cscheid that's great to hear! Yea I think this is a much saner approach. I've also realized that this is a problem in more than Positron #509 (comment), so I've devised TaskQueueManager in such a way that we can use it for all of the other languages' execute() hooks too. Each language will get its own individual task queue.

[lionel-]

I like this new setup!

I think we could use an existing async queue? In the positron-r extension we use PQueue.

[DavisVaughan]

@lionel- it seems like PQueue was actually exactly what we needed. I took a look at their implementation and it seems extremely similar to my queue.ts approach, I basically reimplemented it without knowing. Can you take one more look please?