fix: add pending browser permission state

[myandrienko]

Overview

Previously there was no way to track if the SDK is currently prompting browser for a device permission. For example:

1. Call has camera enabled by default. User visits the call page for the first time. Browser permission prompt is displayed, but in the meanwhile camera toggle is showing that camera is enabled (because hasBrowserPermission is true for prompt state, and optimisticStatus is enabled).
2. Permissions have been reset, and user tries to enable camera. Once again, browser permission prompt is displayed, but in the meanwhile camera toggle is showing that camera is enabled.

We now introduce another permission state apart from prompt, granted and denied: prompting. This state is in effect while browser prompt is visible, i.e. when permission state is prompt and getUserMedia promise is pending.

Implementation notes

New browser permission state prompting is added to BrowserPermission class.

This state is exposed via the isPermissionPending$ observable in input media device manager state.

State hooks for camera and microphone state also return this flag.

Default device toggles support this flag and display the new "?" badge while permission is pending:

Docs are to follow.

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There are also two other minor changes in this PR.

The settled() check in concurrency.ts is fixed to prevent possible race conditions. Previously, it was possible to check for settled(), then add more promises to the queue, and settled() will resolve before these newly added promises are resolved.

isTogglePending flag is returned from device-related call state hooks. It can be used to display a loader while camera/microphone are enabled/disabled. It's not used by our default toggle buttons, but it's nice to have.