Add-pages-functions-build-command (#210)

* Add  command

* Update .changeset/purple-planets-join.md

Co-authored-by: Pete Bacon Darwin <[email protected]>

* Add sourcemap option

Co-authored-by: Pete Bacon Darwin <[email protected]>

Author: GregBrimble

.changeset/purple-planets-join.md

@@ -0,0 +1,7 @@
+---
+"wrangler": patch
+---
+
+Expose `wrangler pages functions build` command, which takes the `functions` folder and compiles it into a single Worker.
+
+This was already done in `wrangler pages dev`, so this change just exposes this build command for use in our build image, or for people who want to do it themselves.