Allow --incremental Builds to process more than one queued file at a time

[zachleat]

Related to #3324, #3077.

> eleventy-base-blog@9.0.0 start
> npx @11ty/eleventy --serve --quiet --incremental

[11ty/eleventy-img] 1 image optimized (1 deferred)
[11ty] Copied 6 files / Wrote 16 files in 0.18 seconds (11.3ms each, v3.0.0-alpha.13)
[11ty] Watching…
[11ty] Server at http://localhost:8080/
[11ty] File changed: content/blog/firstpost.md
[11ty] File changed: content/blog/fourthpost/fourthpost.md
[11ty] File changed: content/blog/thirdpost.md
[11ty] File changed: content/blog/secondpost.md
[11ty/eleventy-img] 1 image optimized (1 deferred)
[11ty] Benchmark      8ms  22%     9× (Configuration) "slugify" JavaScript Function
[11ty] Wrote 2 files (skipped 11) in 0.04 seconds (v3.0.0-alpha.13)
[11ty/eleventy-img] 1 image optimized (1 deferred)
[11ty] Benchmark      7ms  29%     9× (Configuration) "slugify" JavaScript Function
[11ty] Wrote 2 files (skipped 11) in 0.03 seconds (v3.0.0-alpha.13)
[11ty/eleventy-img] 1 image optimized (1 deferred)
[11ty] Benchmark      7ms  32%    12× (Configuration) "slugify" JavaScript Function
[11ty] Wrote 2 files (skipped 11) in 0.02 seconds (v3.0.0-alpha.13)
[11ty/eleventy-img] 1 image optimized (1 deferred)
[11ty] Benchmark      5ms  28%     9× (Configuration) "slugify" JavaScript Function
[11ty] Wrote 2 files (skipped 11) in 0.02 seconds (v3.0.0-alpha.13)

[zachleat]

Moving this one out of the queue #4274 and milestoning for 4.0.0-alpha.8