Adds ondemand support for serve mode

[samthor]

This adds a --ondemand flag that, when combined with --watch, only builds pages which are requested. (This is basically the idea behind 11ty-fast-dev).

In our testing, this provides ~1.5x speedup on test Eleventy sites during development. If you have costly transforms or filters, it speeds it up even more (this is the case for developer.chrome.com, where we unfortunately do HTML parsing and rewriting as part of this step).

If running your templates (and their filters) or transforms has global side-effects and you reply on that during development, then this probably isn't a good idea.

Cons:

• I'm not entirely sure how well this interacts with incremental build
• You might want this behavior without BrowserSync (in fact, developer.chrome.com probably does, but this feature just makes sense behind --watch).

[dgrammatiko]

Suggested change

	let promise = this.writer.write();
	ret = await promise;
	ret = await this.writer.write();

[Ryuno-Ki]

Any updates?

[samthor]

I'm waiting for input from @zachleat.

I suspect his work on #1629 might make this sort of approach easier, but I'm not sure yet.