fix: narrow default node_modules exclude

[chenjiahan]

Summary

This PR narrows the default exclude option so React Refresh skips only real node_modules path segments instead of any path containing the node_modules substring. It also updates the README default and adds coverage for paths whose names merely contain node_modules.

[Copilot]

Pull request overview

This PR refines the plugin’s default exclude behavior so React Refresh ignores only true node_modules path segments (instead of any path containing the substring), and updates documentation/tests to reflect and validate that behavior.

Changes:

• Narrow default exclude from a broad /node_modules/i-style match to a path-segment match (/[\\/]node_modules[\\/]/).
• Add a regression test + fixture to ensure paths whose names merely contain node_modules are still processed.
• Update README to document the new default and example configuration.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/options.ts	Updates the default exclude regex used during options normalization and documents it in JSDoc.
README.md	Updates the documented default exclude and example patterns to match the new behavior.
test/test.spec.ts	Adds coverage for paths that contain node_modules as part of a directory name (not as a segment).
test/fixtures/contains-node_modules-name/index.js	Adds a new fixture used by the regression test.

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