Modify legacy registration mechanism

[jerelmiller]

This is a precursor to some work I'm doing to make client discovery a bit less chaotic. Uses Object.defineProperty for the global property to monitor its setter and register the client with the devtools.

[jerelmiller]

We no longer need this on initialization since we register the client immediately if its available, or registers it when the value is assigned from a legacy client.

[relativeci]

#409 Bundle Size — 1.26MiB (~+0.01%).

5040547(current) vs f6f500e main#394(baseline)

Warning

Bundle contains 12 duplicate packages – View duplicate packages

Bundle metrics   2 changes 1 regression

	Current #409	Baseline #394
Initial JS	1.22MiB(~+0.01%)	1.22MiB
Initial CSS	0B	0B
Cache Invalidation	1.13%	0%
Chunks	5	5
Assets	12	12
Modules	940	940
Duplicate Modules	45	45
Duplicate Code	3.82%	3.82%
Packages	160	160
Duplicate Packages	9	9

Bundle size by type   1 change 1 regression

	Current #409	Baseline #394
JS	1.22MiB (~+0.01%)	1.22MiB
IMG	35.85KiB	35.85KiB
HTML	810B	810B
Other	778B	778B

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Bundle analysis report Branch jerel/register-legacy Project dashboard

[jerelmiller]

This was almost never true, but with the changes in this PR, we don't need it anyways since we will register the client immediately if its available when this content script loads, or register it when the client is assigned on the window.