Support Flow ReadOnlyArray type

[sellimenes]

Summary

• Map Flow's $ReadOnlyArray<T> built-in to the runtime array(T) type instead of falling through to an object/ref conversion.
• Add a regression fixture for the issue example so $ReadOnlyArray<string | number> asserts an array of the expected union type.

Fixes #199

IssueHunt bounty: https://issuehunt.io/r/gajus/flow-runtime/issues/199

Testing

• yarn --cwd packages/flow-config-parser build
• TEST_FILTER='199-support' yarn --cwd packages/babel-plugin-flow-runtime test
• yarn --cwd packages/babel-plugin-flow-runtime build
• yarn --cwd packages/babel-plugin-flow-runtime test -> 238 passing
• yarn --cwd packages/babel-plugin-flow-runtime lint exits 0 with existing jsx-a11y/alt-text warnings
• git diff --check