[Feature]sidePanel messaging

[KevinZhang19870314]

Since chrome have feature sidePanel, I think it is necessary to add this types.

[iplanwebsites]

How did you guys worked around this using webext-bridge?

[adao]

I forked a copy locally and added a side-panel.ts file, see #91

[weijuer]

so, this is dead repo

[EspressoBeans]

👆🏽 this