feat: plausible module

[tikagan]

• plausible middleware refactored into module
• plausible module optionally fires in non-production mode if local tracking explicitly enabled in nuxt.config

[orvn]

@tikagan can you remind me why we added the module files directly to the repo instead of including it as a dependency?

[tikagan]

@orvn two reasons:

1. we haven't packaged this up and made it an npm module yet
2. this version of the module is for Nuxt 2 and the version we're planning on packaging up for npm is for Nuxt 3 (I did look into it but there's no way to write the module so it's compatible with both)