migrate `c12` to `unconfig`

[northword]

Clear and concise description of the problem

bumpp has switched from c12 to unconfig.

Currently, in the scaffold code, the main difference between the two lies in the support for configuration file types and .env file reading.

For configuration files, supporting zotero-plugin.config.m?[jt]s is sufficient.
For .env reading, after #175, we can use loadEnv provided by vite to impl this, and it can also close #72.

Suggested solution

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