Fix #7920 - do not add comma for explicit inexact object with indexer property

[DmitryGonchar]

Fixes #7920 - do not add comma for explicit inexact object with indexer property - for all cases. Previously it was removed only when type had some simple "properties" field defined.

In tests, the main factor that it works is that comma being shown or hidden behaves the same for both the simple/main case

type T = {
  a: number,
  ...,
}

and for object with indexer property or without properties.

• I’ve added tests to confirm my change works.
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