recordWithBrandedKeys should not be Partial

[AlexErrant]

I am attempting to fix a regression. Here's a timeline:

1. This PR describes an issue and is merged. Note that it primarily adds

  : K extends BRAND<string | number | symbol>
  ? Record<K, V>

2. For reasons that are unclear to me this commit reverses the order of the above and uses [] syntax, causing a regression:

  : [BRAND<string | number | symbol>] extends [K]
  ? Record<K, V>

3. My PR switches the order back to 1, keeps the [] syntax, and adds a test to prevent regressions:

  : [K] extends [BRAND<string | number | symbol>]
  ? Record<K, V>

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