feat(wrangler): Implement environment inheritance for Pages (#5234)

To make it easier for users to write config files for Pages,
Wrangler will not require both of those environments to be
explicitly defined in the `wrangler.toml` file. If either
`[env.production]` or `[env.preview]` is left unspecified,
Wrangler will use the top-level Wrangler environment when
targeting that named Pages environment.

This commit implements the aforementioned environment
inheritance logic for Pages projects.

Co-authored-by: Carmen Popoviciu <cpopoviciu@cloudflare.com>

.changeset/fifty-rules-thank.md

@@ -0,0 +1,7 @@
+---
+"wrangler": minor
+---
+
+feat: Implement environment inheritance for Pages configuration
+
+For Pages, Wrangler will not require both of the supported named environments ("preview" | "production") to be explicitly defined in the config file. If either `[env.production]` or `[env.preview]` is left unspecified, Wrangler will use the top-level environment when targeting that named Pages environment.