Preserve `tested-with` formatting from existing `.cabal` file

[webdevred]

Problem:
hpack changes capitalization and spacing in the tested-with section, breaking Gild formatting. Related: #617

Expected behavior:
Maintain exact formatting, including capitalization, spacing, and commas.

Example:

-- Before (Gild-formatted)

tested-with:
  ghc ==9.4.7
  ghc ==9.6.6

-- After (hpack)

tested-with:
  GHC == 9.4.7,
  GHC == 9.6.6