fix(es/plugin): Fix starter template main field (#6814)

[alex-dixon]

Description:
Replace - with _ in generated package.json main field to match what cargo names the output .wasm file. Allows swc-core to load the plugin at runtime.

Related issue:

• Closes Plugin CLI template generates wrong package.json main #6814.

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[kdy1]

Thanks!

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swc-bump:

• dbg-swc