feat(sustainable) : added countryCode option for country specific CO₂ calculations

[Harsh-Git39]

Makes users to calculate CO₂ emissions using a country-specific electricity grid mix via electricitymaps (through @tgwf/co2).

Implementation

Added sustainable.countryCode as a plugin CLI option.
Passed the value into the CO₂ model configuration when creating the co2 instance.
Updated the sustainable plugin documentation with a implementation example .

Testing and observations

I tested it with different country codes (IN, DE, NO). On very small pages (example.com) the difference is tiny due to rounding, but on larger pages like wikipedia.org the CO₂ values do change (for example, India ≈ 0.03730 vs Norway ≈ 0.03710), which shows the country-specific electricity mix is being used.
On lightweight pages (like example.com / wikipedia), the differences in CO₂ values are small, but they are consistently visible across runs, which suggests the country-specific electricity mix is being applied correctly. Heavier pages should make the differences more noticeable.

Thanks a lot for opportunity!!

[soulgalore]

Hi @Harsh-Git39 ah cool, I'll have a look and test it this weekend!

[Harsh-Git39]

Hi @soulgalore! I've fixed all the linting errors. The CRUX test is failing due to a missing API key in the CI configuration (--crux.key is empty), which appears to be unrelated to my changes. All other tests are passed . Could you please review?