feat(gatsby): Add eslint rules to warn against bad patterns in pageTemplates (for Fast Refresh)

[LekoArts]

Description

This adds new ESLint rules to our default configuration that enforce two things in page templates (so src/pages/* and pages created with createPage):

• No anonymous default exports
• Page templates must only export one default export (the page) and query as a named export

Additionally we also re-enabled the pascalCase rule (as it doesn't seem to trip up <Styled.p> from theme-ui anymore.

This all happens for Fast Refresh as it requires these code patterns to work correctly and without a full hard reload.

This PR will also include #28911

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Old comment about Babel Plugin

This adds babel plugin to `develop` stage when fast-refresh is enabled that checks wether page template is hot reloadable with fast-refresh: - page template (default export) need to be named component (and it need to start with uppercase actually) - page template must not have additional exports (other than page query)

Messages in Console:

TODO:

• make warning messages more actionable and in general worded better ( current ones are for most part just "placeholders" to make sure this works )
• print warnings also in browser console (this is already handled)
• find actually proper place to make those checks - babel plugin is/was good starting point because we get the AST, but there are problems with it:
  • this is in fact lint rule and not transformation (unless we make it automatic transformation, but that is risky, plus it's not always possible - like multiple exports can't be transformed into something working with fast refresh). So this is really better suited to be run with eslint loader ... but users can disable builtin eslint configuration very easily and use custom one with gatsby-plugin-eslint. We don't "merge" eslint options and don't have requiredRules, defaultRules kind of deal in same way we have for babel
  • because this is babel plugin - it will reuse cached results, meaning if we have page template resulting in warning, do edit -> save -> rollback to previous file content - it will reuse already cached thing without reprinting the warning. We could throw errors/warnings in the plugin but that would completely fail compilation and I'm not sure if this is very user friendly. Babel plugins doesn't seem to have builtin error/warning tracking (not sure here - I just couldn't find anything about this)
  • we could create webpack loader and use webpack's warning mechanism, but this might mean we would need to parse code from string (into AST) again - but this should work around the caching problems - and also actually work well when it comes to showing the warning both in terminal and in browser console

Related Issues

[ch19773]
[ch22489]
[ch22491]

[pvdz]

I think it's fine to go? Please do go through the comments. Most things are improvements rather than problems. Some things are only problems if you say they are :)

[pvdz]

Two cases are nits so I'd say merge with or without them applied.