fix(scheduler): prevent duplicate jobs being queued

[skirtles-code]

Fixes #11712
Fixes #11807

I still need to write some tests for this, but the existing tests pass.

A job should only appear in the active part of the scheduler queue once. Currently it can be added multiple times, with the job being added again every time a reactive dependency changes.

The QUEUED flag should be set for all jobs that are currently in the queue, not just those with a particular value for ALLOW_RECURSE.

The ALLOW_RECURSE flag is only relevant if a job is re-queued while that same job is currently running. Here I've implemented that by clearing the flag QUEUED immediately prior to running the job if ALLOW_RECURSE is set. Otherwise the flag is cleared after the job runs.

It isn't clear to me how errors should be handled. For the main queue, I assume the QUEUED flags all need to be unset in the finally block, as an error may have been thrown during flushing that prevented some of the flags being unset. For the post queue there doesn't seem to be equivalent error handling and I'm not sure if that should be added.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB (+103 B)	37.8 kB (+21 B)	34 kB (+21 B)
vue.global.prod.js	159 kB (+103 B)	57.6 kB (+20 B)	51.2 kB (-19 B)

Usages

Name	Size	Gzip	Brotli
createApp	55.3 kB (+103 B)	21.3 kB (+24 B)	19.4 kB (-39 B)
createSSRApp	59.3 kB (+103 B)	22.9 kB (+25 B)	20.9 kB (-12 B)
defineCustomElement	60 kB (+103 B)	22.8 kB (+24 B)	20.8 kB (+65 B)
overall	69.1 kB (+103 B)	26.4 kB (+24 B)	24 kB (+21 B)

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commit: c23c01f

[yyx990803]

Great work, I'll get this released in a patch, let's follow up with a test case later.