Partials are currently defined via MDX includes. This PR switches to pre-compile-time partials, which have a new syntax:
```
<$Partial path="path/to/file.mdx" />
```
## Rationale
This produces two improvements:
1. Partial substitution can occur in pipelines that don't use MDX compilation. For example, we can now do partial substitution before building the search index, so partial content will also be indexed.
2. After the App Router migration, the MDXProviders should've been deprecated, but were kept around for the sole reason of making partials work, and leading to us shipping unnecessary client-side code. We get a minor decrease in overall client bundle size (5.74 MB to 5.6 MB) by getting rid of the Providers.
## Breaking changes
Besides the change to partial syntax, the arguments are also less powerful than before because we are doing string substitution and don't have the full power of JS. Defining string variables is still possible (documented in the Contributing guide), and since that's all we actually do in practice, this shouldn't be too cumbersome. There is always the escape hatch of making a custom component for more complex content reuse cases.
* Check all env vars for studio and list them properly in turbo.json.
* Check all env vars for design-system.
* Fix all env vars for www.
* Fix all env vars for docs.
* Add the rest of the env vars from the packages folder.
* Revert the changes in the sign-in pages.
* Remove useless files. Fix a comment.
* Fix the links in extensions page to always point to supabase.com/docs.
* Change the feature flag rewrite in studio to point to supabase.com/.flags endpoint.
* Replace NEXT_PUBLIC_VERCEL_URL with NEXT_PUBLIC_VERCEL_BRANCH_URL because it's more stable on vercel.com. Remove it from apps where it's not needed.
* Add env vars from packages. Move all passthrough env vars to env vars since they're stable (don't change between pushes).
* Revert changes of studio files.
* Minor fixes for the merge commit.
* Replace all uses of SITE_URL with LW_URL (they're the same). SITE_ORIGIN is not constructed from env vars. LW_URL is constructed from SITE_ORIGIN and launch_week string.
* Use SITE_ORIGIN when fetching the ticket-og.
We allow fetching external data in CodeSamples into a MDX environment, so we have to be careful about preventing code execution.
Current checks:
- External data is inserted as a code block (via the AST, not direct string manipulation), so it is escaped.
Added two new layers of checks:
- Allow-list of organizations, currently set to Supabase-only
- Only allow immutable commit references
Add the ability to embed code samples from GitHub into tutorials, so we can have a single source of truth for the source code.
Introduces the <$CodeSample /> syntax, which is a special syntax and not a real React component (see the directives/README.md for why on earth I did it this way -- in this specific case, CodeHike adjusts the MDX syntax tree before it gets compiled, and we need to adjust it ourselves before CodeHike sees it, so we need to get down to the level of manipulating the AST in order to make this work with CodeHike).
Adjusted one of the example tutorials to use this new feature as a test.
