refactor native beforeinput handling

[ianstormtaylor]

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

Debt.

What's the new behavior?

Moves the native beforeinput event handling into the Before/AfterPlugin and out of the <Content> component, so that all event handling remains in plugins.

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: #2059

[codecov]

Codecov Report

Merging #2063 into master will decrease coverage by 0.07%.
The diff coverage is 23.61%.

@@            Coverage Diff             @@
##           master    #2063      +/-   ##
==========================================
- Coverage   66.94%   66.86%   -0.08%     
==========================================
  Files          68       68              
  Lines        5624     5635      +11     
==========================================
+ Hits         3765     3768       +3     
- Misses       1859     1867       +8

Impacted Files	Coverage Δ
packages/slate-react/src/plugins/after.js	5.3% <0%> (-0.68%)	⬇️
packages/slate-react/src/components/content.js	27.35% <0%> (+5.71%)	⬆️
packages/slate-react/src/plugins/before.js	11.57% <0%> (-0.13%)	⬇️
packages/slate-dev-environment/src/index.js	51.35% <48.57%> (-3.83%)	⬇️

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