Provide consistent access to `browser.permissions` API in the devtools

[twschiller]

Context

The availability of the permissions.request API in developer tool pages is implemented inconsistently across Firefox and Chrome

Browser status:

• Chrome: ✓ supported
• Firefox: x not supported
• Safari: ? (not checked)

Reference:

• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/permissions/request
• https://github.com/pixiebrix/pixiebrix-extension/issues/680

[xeenon]

WebKit Bug

[Rob--W]

Supportive of exposing extension APIs to devtools panel, https://bugzilla.mozilla.org/show_bug.cgi?id=1796933