[adapter-cloudflare-workers] add es2020 target to esbuild function to solve Unexpected character '#' error

[jacob-8]

Fixes #1220 with the one-line fix proposed in issue: #1220 (comment) - this bug is a showstopper for using CI to publish as the manual edit workaround doesn't get committed.

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[Rich-Harris]

thank you! mind adding a changeset so we can release this? just do pnpx changeset and follow the prompts