[router] Support platform extensions for _layout and routes

[marklawlor]

Why

Revised version #26047
Blocked by #27407

How

Rules:

• API routes cannot have platform extensions
• Layouts and routes can have extensions only if a non-extension version exists

For example, you have have /page.web.tsx only if /page.tsx also exists. This makes platform extensions an additive change and won't break concepts like universal deep linking and typed routes

API routes do not support platform extensions as they are independent of a platform.

Implementation

getRoutes allows multiple files to be assigned to the same route, but all files were given the same specificity.

This PR adds logic to calculate the specificity of a file and to select the most specific file.

Specificity highest-to-lowest

2: Platform extension matches platform
1: .native platform extension
0: No platform extensions
-1: Invalid route

If no platform is provided to getRoutes the current behaviour is retained and files with platform extensions will be ignored.

getRoutes checklist

These are the locations we use getRoutes and whether they should use platform extensions or not.

renderStaticContent

• Used in getManifest() ❌
• Used in getBuildTimeServerManifestAsync ❌ - I'm not 100% sure on this one. Need to understand how to test this correctly

routes-manifest

• Used in createRoutesManifest ❌

router-store

• Used in RouterStore ✅

typed-routes

• Used in getTypedRoutesDeclarationFile ❌

There is also an getExactRoutes function, but this is only used for testing purposes.

**

Test Plan

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[marklawlor]

Add flag to disable.
Merge into next SDK