Right now we create the SSR serverless function at path `_ssr`, and then symlink all the other pages to that function.
Instead just make the first page encountered be the "real" function, and symlink all the other pages to that endpoint.
Previously, the page-data Serverless Function and SSR Serverless Function were two distinct functions. They had almost identical file contents and just slightly different handler logic. So here we merge the handler logic into a single function and re-use the same Serverless Function for both page-data and SSR.
This simplifies the output quite a bit and deletes a good amount of code, and helps with build output size, cold boot times, etc.
* Sets a valid number of seconds for DSG expiration (10 minutes - do we want to make that configurable somehow?)
* Sets the `group` of DSG pages, so that the page-data and SSR pages are associated
* Outputs SSR/DSG pages with `/index.html` suffix so that those paths are accessible
* Updates SSR and page-data functions URL path parsing logic to handle querystrings and the `index.html` scenario
* Remove the unnecessary `rewrite` related to page-data URL paths
* Remove the page-data function static file fallback handling (they are accessible as just regular static files)
* Correct the path for the page-data endpoing when the root-level index page is SSR/DSG
