TextFormatRuns: Runs of rich text applied to subsections of the cell.
Runs are only valid
on user entered strings, not formulas, bools, or numbers. Runs start at specific indexes in the text and continue until the next run. Properties of a run will continue unless explicitly changed in a subsequent run (and properties of the first run will continue the properties of the cell unless explicitly changed).
When writing, the new runs will overwrite any prior runs. When writing a new user_entered_value, previous runs will be erased.