Add flow-runtime webpack loader

[landeqiming666]

Closes #165.

Summary

• add a new flow-runtime-loader package for importing *.js.flow type declarations as flow-runtime runtime validators
• pass loader options through to babel-plugin-flow-runtime, with a nested babel option for Babel transform configuration
• support webpack 4 getOptions usage, legacy query options, async and synchronous loader execution, and source map passthrough
• document inline loader usage and webpack rule usage

Tests

• npm run lint in packages/flow-runtime-loader
• npm test in packages/flow-runtime-loader (6 passing, including a webpack fixture that imports a .js.flow file through the loader)
• npm run build in packages/flow-runtime-loader

Notes

• I had to build local workspace dependencies (babel-plugin-flow-runtime and flow-config-parser) before running the new package tests in a fresh checkout.
• A plain npm install on Node 25 fails in this old repository because node-sass@4 tries to compile with an old node-gyp / Python setup; npm install --legacy-peer-deps --ignore-scripts was sufficient for this focused validation.