Add code block language hints

[shriram-svg]

Summary

• show code block language hints as soon as a user types a backtick or tilde fence opener
• add a plain code block option so users can skip choosing a language
• complete the selected hint into a full fenced code block and place the cursor inside it
• cover the new code block hint behavior with focused Jest tests

Closes #1140.

Verification

• npx npm@6 run lint
• npx npm@6 run test-api -- --runInBand
• ./node_modules/.bin/tsc --noEmit --pretty false
• ./node_modules/.bin/prettier --check src/cloud/components/Editor/index.tsx src/cloud/lib/editor/CodeMirror.ts src/cloud/lib/editor/CodeMirror.spec.ts
• git diff --check

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