Write templates synchronously

[shivjm]

I’m not sure how to implement a queue to resolve #3271 but here’s my proposed workaround. This does block everything while writing a template (hopefully a small window).

Closes #3271
Closes #2627

[shivjm]

@uncenter pointed out that the Fetch and Image plugins use the p-queue library. I think I’ll add a commit to this PR using p-queue; if it’s not desirable, I can back it out.

[zachleat]

I’m okay with trying this out in 3.0.0-alpha.11.

Preliminary benchmarking suggests that the performance cost is about the same here:

---------------------------------------------------------
Eleventy Benchmark (Node v20.12.2, 10000 templates each)
---------------------------------------------------------
Eleventy 3.0.0-alpha.10                                        
---------------------------------------------------------
.md: ... 3 runs:              
* Median: 7.92 seconds
* Median per template: 792 µs

---------------------------------------------------------
Eleventy 3.0.0-alpha.11                                        
---------------------------------------------------------
.md: ... 3 runs:              
* Median: 8 seconds (1%)
* Median per template: 800 µs (1%)

[zachleat]

Somewhat related (but for the rendering pipeline, not file writing): #3277.

Happy to entertain some other queuing additions here if we think it would be helpful! Though https://www.npmjs.com/package/graceful-fs does some of this already!