SF-3212 Create 'unknown' blot as fallback blot

[siltomato]

This PR creates a custom ScrollBlot to override Quill's scroll blot and an UnknownBlot to render when an unknown blot type is attempted.

If an unknown blot type creation is attempted, the custom ScrollBlot catches the error and creates the UnknownBlot. After the editor renders (via setTimeout()), an error is thrown displaying the name of the failed attempted blot.

UnknownBlot uses tagName = 'sf-unknown', ('span' was causing it to be incorrectly matched from a quill registry query and throw an after.appendChild not a function error).

You can test this blot by adding TNN as a resource.

Example:

---

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 82.71%. Comparing base (dc4f90a) to head (5abc7cc).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...l-editor-registration/quill-formats/quill-blots.ts	53.33%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3052      +/-   ##
==========================================
- Coverage   82.72%   82.71%   -0.02%     
==========================================
  Files         563      563              
  Lines       32653    32669      +16     
  Branches     5282     5284       +2     
==========================================
+ Hits        27012    27021       +9     
- Misses       4862     4868       +6     
- Partials      779      780       +1     

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[josephmyers]

I haven't done a full review yet, but I think it's less than ideal to utilize the generic exception dialog for this, since that's typically reserved for errors that aren't handled, as a last resort. Per the JIRA description, we want to be notified of new blots that we need to support; however, we can be a little more elegant than the last-resort exception dialog. You could:

• Log an error to the console for the least intrusion
• Show an error "notice" as a middle ground

I'd think the former, since a dialog or even an error notice showing every single time would get pretty old pretty fast to users. What do you think?

[siltomato]

Yes, I think you are right. The error dialog does get old fast. The idea was to alert (still rendering the text) so that the missing blot could be remedied, but perhaps the visual presence in the text is sufficient (along with logging to the console).

Reviewable status: 0 of 4 files reviewed, all discussions resolved (waiting on @josephmyers)

[josephmyers]

Thanks. I've tested this with resources and projects, and it seems to handle things as expected. Awesome!

Reviewed 3 of 4 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @siltomato)