Fix Astro components parent-child render order

[bluwy]

Changes

Fix #8059

Previously, the renderComponent API will kick off the render directly, however I didn't notice that component slots are inited eagerly (for head propagation) and it kicked off the rendering of slots eagerly (not good).

This PR reverts to the previous technique where the parent will render each child in parallel manually instead. It renders all the child into a buffer first, then flush it out when it's its turn to render.

NOTE: While the benchmark shows that this is faster, it's because I removed the bufferChunks.length = 0 optimization which seems to do more harm than good for perf. If I ported that, the perf is actually slightly lower because things are less in parallel.

Testing

Added a new test

Docs

n/a. bug fix.

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Latest commit: 26fa3e4

The changes in this PR will be included in the next version bump.

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[bluwy]

!bench server-stress

[github-actions]

PR Benchmark

Server stress

	Avg	Stdev	Max
Latency	2321.46 ms	619.93 ms	4433 ms

	Avg	Stdev	Min	Total in 30s
Req/Sec	417.07	156.28	291	12.5k requests
Bytes/Sec	29.6 MB	11.1 MB	20.7 MB	889 MB read

Main Benchmark

Server stress

	Avg	Stdev	Max
Latency	2406.12 ms	423.84 ms	3707 ms

	Avg	Stdev	Min	Total in 30s
Req/Sec	400	177.6	370	12.0k requests
Bytes/Sec	28.4 MB	12.6 MB	26.3 MB	853 MB read