Add prepack script (#57)

In #54, I removed the `postinstall` script. Our build/publish
pipeline actually depended on the build that was a consequence of
the `postinstall` script, so this broke publishing in that the
build never occurred.

This commit adds a `prepack` script which runs the build prior to
packing for publish.

.changeset/many-ears-act.md

@@ -0,0 +1,5 @@
+---
+'@apollo/datasource-rest': patch
+---
+
+Fix build process (again), ensure built directory exists before publish

package.json

@@ -21,6 +21,7 @@
  "build": "tsc --build tsconfig.build.json",
  "clean": "git clean -dfqX",
  "install-with-npm-8.5": "npm i -g npm@^8.5.0 && npm i",
+ "prepack": "npm run build",
  "prettier-check": "prettier --check .",
  "prettier-fix": "prettier --write .",
  "publish-changeset": "changeset publish",