Upgrade miniflare to 2.0.0 🎉 (#221)

* chore: upgrade `miniflare` to `2.0.0`

* fix: remove redundant immediate `pages dev` rebuilds/reloads

Watching `all` events without `ignoreInitial` set to `true` means
`chokidar` will call the callback immediately. This caused functions
to be built twice, and Miniflare to be `reload`ed immediately after
startup.

* fix: wait for `esbuild` to finish before starting Miniflare

Instead of using a `scriptReady` boolean, this now uses a deferred
`Promise`, so the event loop isn't blocked. This is `await`ed before
constructing the `new Miniflare`, as this will asynchronously kick
off loading the script.

* fix: undefined fallback fetch on watch update with `pages dev`

Subsequent Pages Functions worker builds due to file changes were
not replacing `__FALLBACK_SERVICE_FETCH__` leading to
`ReferenceError`s. This change switches to using `esbuild`'s built-in
`define` option, instead of `esbuild-plugin-replace`, removing an
additional dependency.

.changeset/grumpy-knives-bathe.md

@@ -0,0 +1,5 @@
+---
+"wrangler": patch
+---
+
+Upgrade `miniflare` to `2.0.0`