browser.secureStorage: Create polyfill for implementation

[oliverdunk]

As mentioned at the last WECG meeting a polyfill would be a good way for people to experiment with the secureStorage API and get a feel for if the API surface works in their extension. We should look at building something based on what we have today, possibly alongside the proposal in this repo.

[oliverdunk]

We won't be able to actually use secure storage here, so it will likely use something like local storage and print a warning that this isn't production safe.