Fix tests typo, add Cargo.toml to .npmignore

[ShukantPal]

I noticed that the test files are still included in the published npm package at, e.g. https://www.npmjs.com/package/@swc/plugin-relay?activeTab=code

I'm assuming that's a typo.

Additionally, I added Cargo.toml to the .npmignore to help avoid conflicts with multi-language projects since Cargo can read into node_modules, e.g. bazelbuild/rules_rust#3425

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