feat: build.watch_dir can be an array of paths (#1231)

In projects where:

- all the source code isn't in one folder (like a monorepo, or even where the worker has non-standard imports across folders),
- we use a custom build, so it's hard to statically determine folders to watch for changes

...we'd like to be able to specify multiple paths for custom builds, (the config `build.watch_dir` config). This patch enables such behaviour. It now accepts a single path as before, or optionally an array of strings/paths.

Fixes #1095

.changeset/stupid-dots-beam.md

@@ -0,0 +1,14 @@
+---
+"wrangler": patch
+---
+
+feat: `build.watch_dir` can be an array of paths
+
+In projects where:
+
+- all the source code isn't in one folder (like a monorepo, or even where the worker has non-standard imports across folders),
+- we use a custom build, so it's hard to statically determine folders to watch for changes
+
+...we'd like to be able to specify multiple paths for custom builds, (the config `build.watch_dir` config). This patch enables such behaviour. It now accepts a single path as before, or optionally an array of strings/paths.
+
+Fixes https://github.com/cloudflare/wrangler2/issues/1095