In the past, we used the `VERCEL_ANALYTICS_ID` environment variable with the previous Speed Insights feature on Vercel to activate specific logic in Next.js, Nuxt and Gatsby for collecting and sending web vitals data.
With the new Speed Insights, that's not required anymore.
We no longer want to set the environment variable when detecting the new `@vercel/speed-insights` package.
This PR confirms that the variable is not set if the new package is detected.
### 🧐 What's in there?
With the recent release of [`@vercel/speed-insights`](https://vercel.com/docs/speed-insights/package) own package (like [`@vercel/analytics`](https://vercel.com/docs/analytics/package)), it's time to encourage users to migrate.
With the availability of `@vercel/speed-insights`, users will have to opt-in explicitly by installing the package. Their benefit is a better and fine-grained control of the reporting (in particular, per-application sample rate).
### ❗ Note to reviewers
I used `console.warn` and hope it will stands out in the build logs. I'm happy to use anything else if you have better suggestions.
There's also a deprecation warning in Next.js, which covers a related but slightly different case, when users explicitly pass analyticsId in their configuration. https://github.com/vercel/next.js/pull/60677
Updates the `static-build` injector to use the new plugin.
Still need to verify somehow that the newly published plugin is working as expected. It should be fine since it was a copy-paste from the previous plugin repo, but always good to verify before we break everything!
This PR also updates the README in `@vercel/gatsby-plugin-vercel-analytics`
Adds support for auto injection into Gatsby's new ESM config format. Only injects if the `gatsby-config.mjs` is present, and continues to default generation of `gatsby-config.js`
### Related Issues
A customer issue was raised that web-vitals analytics were not working for their Gatsby application, even though this is meant to be zero config.
It turns out the issue is due to their `gatsby-config` file being declared as a `ts` file, rather than `js`. This is perfectly valid and supported in Gatsby.
However, the static-build modifications that are made to automatically add the `gatsby-plugin-vercel` only apply to existing `js` files.
This lead to their deployments containing both a `js` and `ts` version of `gatsby-config`. Luckily, the `ts` version has higher precedence, so _only_ web-vitals were affected.
Closes https://linear.app/vercel/issue/FLA-364/investigate-gatsby-and-nuxt-data-issues
#### Tests
- [X] The code changed/added as part of this PR has been covered with tests
- [X] All tests pass locally with `yarn test-unit`
#### Code Review
- [X] This PR has a concise title and thorough description useful to a reviewer
- [X] Issue from task tracker has a link to this PR
