refactor slate-hotkeys, fix deleting at start of block

[ianstormtaylor]

Is this adding or improving a feature or fixing a bug?

Bug & debt.

What's the new behavior?

Refactor's the slate-hotkeys package to make it easier to reason about, and fix an issue that was present in the delete hotkeys on macs.

How does this change work?

Use a simpler map of hotkeys instead of writing functions inline. This is possible thanks to the new version of is-hotkey with optional hotkeys and multiple hotkeys features.

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Fixes: #1887
Fixes: #1888
Fixes: #1983

[codecov]

Codecov Report

Merging #2048 into master will decrease coverage by 0.12%.
The diff coverage is 54.83%.

@@            Coverage Diff             @@
##           master    #2048      +/-   ##
==========================================
- Coverage   67.06%   66.94%   -0.13%     
==========================================
  Files          68       68              
  Lines        5660     5624      -36     
==========================================
- Hits         3796     3765      -31     
+ Misses       1864     1859       -5

Impacted Files	Coverage Δ
packages/slate-react/src/plugins/after.js	5.98% <0%> (ø)	⬆️
packages/slate-react/src/plugins/before.js	11.7% <0%> (ø)	⬆️
packages/slate-hotkeys/src/index.js	80.95% <80.95%> (-3.26%)	⬇️

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