Refine typedef for bindings.vars

[1000hz]

What this PR solves / how to test:
This PR fixes a TypeScript issue that cropped up while updating the miniflare dependency following cloudflare/miniflare#635 fixing its exported types. This change refines the type from unknown to a more accurate string | Json type.

Associated docs issue(s)/PR(s):

N/A

Author has included the following, where applicable:

• Tests
• Changeset (Changeset guidelines)

Reviewer is to perform the following, as applicable:

• Checked for inclusion of relevant tests
• Checked for inclusion of a relevant changeset
• Checked for creation of associated docs updates
• Manually pulled down the changes and spot-tested

[changeset-bot]

🦋 Changeset detected

Latest commit: 67a8b9d

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

This PR includes changesets to release 1 package

Name	Type
wrangler	Patch

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[github-actions]

A wrangler prerelease is available for testing. You can install this latest build in your project with:

npm install --save-dev https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5672204954/npm-package-wrangler-3678

You can reference the automatically updated head of this PR with:

npm install --save-dev https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/prs/5672204954/npm-package-wrangler-3678

Or you can use npx with this latest build directly:

npx https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5672204954/npm-package-wrangler-3678 dev path/to/script.js

Additional artifacts:

npm install https://prerelease-registry.devprod.cloudflare.dev/workers-sdk/runs/5672204954/npm-package-cloudflare-pages-shared-3678

Note that these links will no longer work once the GitHub Actions artifact expires.

[codecov]

Codecov Report

Merging #3678 (67a8b9d) into main (d4450b0) will decrease coverage by 0.01%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3678      +/-   ##
==========================================
- Coverage   75.03%   75.02%   -0.01%     
==========================================
  Files         194      194              
  Lines       11287    11301      +14     
  Branches     2970     2977       +7     
==========================================
+ Hits         8469     8479      +10     
- Misses       2818     2822       +4     

Files Changed	Coverage Δ
packages/wrangler/src/api/dev.ts	82.08% <ø> (ø)
...src/deployment-bundle/create-worker-upload-form.ts	88.13% <ø> (ø)
packages/wrangler/src/deployment-bundle/worker.ts	100.00% <ø> (ø)
packages/wrangler/src/dev.tsx	83.95% <ø> (ø)

... and 7 files with indirect coverage changes