Port over GHC#15115 to text

[hvr]

NB: this will require a major version bump

[phadej]

https://gitlab.haskell.org/ghc/ghc/issues/15115 is Numeric.showEFloat does not honor precision with 0 digits

[Bodigrim]

Sadly we missed this for text-2.1 and I would hope to have a year or two without major bumps.