refactor: Move package.json and other front-end collateral into 'site' (#128)

This refactors the front-end collateral to all live within `site` - so no `package.json` at the root.

The reason we had this initially is that the jest test run and NextJS actually require having _two_ different `tsconfig`s - Next needs `jsx:"preserve"`, while jest needs `jsx:"react"` - we were using `tsconfig`s at different levels at the hierarchy to manage this.

I changed this behavior to still use two different `tsconfig.json`s, which is mandatory - but just side-by-side in `site`.

Once that's fixed, it was easy to move everything into `site`

Follow up from: https://github.com/coder/coder/pull/118#discussion_r796244577

Makefile

@@ -26,9 +26,9 @@ fmt/prettier:
 	@echo "--- prettier"
 # Avoid writing files in CI to reduce file write activity
 ifdef CI
-	yarn run format:check
+	cd site && yarn run format:check
 else
-	yarn run format:write
+	cd site && yarn run format:write
 endif
 .PHONY: fmt/prettier
 
@@ -74,6 +74,6 @@ provisionersdk/proto: provisionersdk/proto/provisioner.proto
 .PHONY: provisionersdk/proto
 
 site/out: 
-	yarn build
-	yarn export
+	cd site && yarn build
+	cd site && yarn export
 .PHONY: site/out