Set a default screen size for the JS driver

[elia]

Summary

Set a default screen size for the JS driver and allow it to be overridden by the example metadata.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov[bot]]

Codecov Report

Merging #353 (f029484) into main (8a32a2a) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #353   +/-   ##
=======================================
  Coverage   97.30%   97.31%           
=======================================
  Files         187      187           
  Lines        4498     4503    +5     
=======================================
+ Hits         4377     4382    +5     
  Misses        121      121           

Files	Coverage Δ
.../spec/support/solidus_starter_frontend/capybara.rb	70.58% <100.00%> (+1.83%)	:arrow_up:

... and 2 files with indirect coverage changes

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