Add reusable ToastMessages component

[acelaya]

Closes #1075

This PR brings the ToastMessages component from client/via, and adds the corresponding documentation to the pattern library.

Once this is merged, we can start using it in frontend apps and remove duplicated code.

Testing steps

1. Check out this branch.
2. Start dev server (make dev)
3. Go to http://localhost:4001/feedback-toast-messages

[acelaya]

Brought verbatim from client https://github.com/hypothesis/client/blob/2fa444e05d806893606a9181423d9d4f35bfbc7f/src/shared/components/ToastMessages.tsx

[acelaya]

Brought verbatim from client https://github.com/hypothesis/client/blob/2fa444e05d806893606a9181423d9d4f35bfbc7f/src/shared/components/test/ToastMessages-test.js

I only added a small fix to make sure there are no components left in the DOM that have side effects in other tests.

[codecov]

Codecov Report

Merging #1272 (ca64c11) into main (217e4e4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1272   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           61        62    +1     
  Lines          844       895   +51     
  Branches       332       344   +12     
=========================================
+ Hits           844       895   +51     

Files	Coverage Δ
src/components/feedback/ToastMessages.tsx	100.00% <100.00%> (ø)

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[robertknight]

LGTM with some minor comments. I would leave the ones about changing the arguments to the onDismiss callback for a subsequent revision.

[robertknight]

The fact that we pass a list of ToastMessages as input, but only emit the id in onMessageDismiss feels a little incoherent and is mainly a relic of use in the client. It would probably make more sense to emit the ToastMessage or index itself. Probably best to get the initial shared iteration landed first though.

[acelaya]

I'll create a follow-up issue to change this for next major version

[acelaya]

#1273

[robertknight]

I had a few minor corrections and comments, otherwise looks good.