Implement GetAvailability for Bluetooth
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This implements the [getAvailability](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetooth-getavailability) function from the spec.
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Reorder startNotifications steps
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https://github.com/WebBluetoothCG/web-bluetooth/pull/355 changed the step order in startNotifications.
The connection check is now Step 2 instead of Step 6.
Link for the current spec state: https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothremotegattcharacteristic-startnotifications
Step annotations are also updated for this function.
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-added parsing and serialization for grid-{row,column}-gap
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I have added an implementation for grid-{row,column}-gap based on the predefined type property suggestion in the description
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This is really part of the "recalc" rather than the "compute" work. But more to
the point, it lets us early-return on the style-sharing stuff (see the next patch).
use `::parse_non_negative instead` of `::parse` for background_size property parsing
Use `::parse_non_negative` instead of `::parse` for background_size property parsing
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