[stable31] Bring back and refactor keymap, add focus trap to plain text editor

[backportbot]

Backport of #7204

[codecov]

Codecov Report

Attention: Patch coverage is 85.41667% with 28 lines in your changes missing coverage. Please review.

Project coverage is 59.28%. Comparing base (15f2e7f) to head (514b4f8).
Report is 13 commits behind head on stable31.

Files with missing lines	Patch %	Lines
src/nodes/CodeBlock/codeBlockShortcuts.js	59.57%	19 Missing ⚠️
src/extensions/Keymap.js	84.31%	8 Missing ⚠️
src/components/PlainTextReader.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## stable31 #7242 +/- ##
============================================
+ Coverage 52.20% 59.28% +7.07% 
============================================
Files 286 287 +1 Lines 40798 36131 -4667 Branches 817 835 +18 ============================================
+ Hits 21300 21420 +120 + Misses 19382 14596 -4786 + Partials 116 115 -1 

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