Add require runtime import option

[dicnunz]

Fixes https://github.com/gajus/flow-runtime/issues/207\n\nIssueHunt bounty: https://issuehunt.io/r/gajus/flow-runtime/issues/207\n\n## What changed\n- Added a new libraryImport option for babel-plugin-flow-runtime, defaulting to the existing ES module import output.\n- When libraryImport: 'require' is set, the plugin emits const t = require('flow-runtime') instead of import t from 'flow-runtime'.\n- Reuses an existing required runtime binding instead of adding a duplicate import, and keeps libraryName working for custom runtime packages.\n- Documented the new option.\n\n## Tested\n- yarn --cwd packages/babel-plugin-flow-runtime test\n- yarn --cwd packages/babel-plugin-flow-runtime build\n- yarn --cwd packages/babel-plugin-flow-runtime lint (exits 0 with existing jsx-a11y rule warnings)\n- git diff --check

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #207: Use require instead of import

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