feat: option for custom fallback instead of `process.env`

[james-elicx]

This PR does the following:

• Makes it possible to provide a custom fallback as an argument. e.g. binding('MY_KV', { fallback: platform.env })

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 8893c5880ad864d4373bda7cd25dd1bf4de092eb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ----------------- | ----- | | cf-bindings-proxy | Minor |

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[github-actions[bot]]

🧪 A prerelease is available for testing 🧪

You can install this latest build in your project with: ```sh npm install --save-dev https://prerelease-registry.ixion-labs.workers.dev/james-elicx/cf-bindings-proxy/runs/5925367823/npm-package-cf-bindings-proxy-25 ``` Or you can immediately run this with `npx`: ```sh npx https://prerelease-registry.ixion-labs.workers.dev/james-elicx/cf-bindings-proxy/runs/5925367823/npm-package-cf-bindings-proxy-25 ```