fix(#6708): change domain order for union with

[ZacharyBys]

fix #6708

Small change to domain order with union with. This was needed to provide custom color mappings when the entire domain isn't known, since the specified fields need to be first to map deterministically to their range values.

Please:

• Make the pull requests (PRs) atomic (fix one issue at a time). Multiple relevant issues that must be fixed together? Make atomic commits so we can easily review each issue.
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• Mention relevant issues in the description (e.g., Fixes #1 / Fixes part of #1).
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  • Add new unit tests.
  • Update the documentation under site/docs/ + add examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
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📦 Published PR as canary version: 5.5.1--canary.8451.0f10787.0

✨ Test out this PR locally via:

npm install vega-lite@5.5.1--canary.8451.0f10787.0
# or 
yarn add vega-lite@5.5.1--canary.8451.0f10787.0

[kanitw]

@ZacharyBys I've included issue number in the conventional commit message and PR description. Please make sure to include them next time.