fix(gatsby): warn if trying to create a page with similar path for trailing/non-trailing

[Moocar]

Description

Warn if trying to create a page whose path is the same as an existing page, but with or without a trailing slash. See #13464

Related Issues

• addresses Warn if creating a page with same path but trailing slash #13464

[DSchau]

Looking good! Worth adding a test for this behavior, perhaps?

[KyleAMathews]

👌