Switch to `fmtTrim` by default

[guibou]

GHC 9.12 introduced multi-line string, see https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/multiline_strings.html. By default they are doing whitespace trimming.

I propose to change PyF in order to:

• Deprecated the str quasiquote in GHC >=9.12
• Change the whitespace trimming of PyF so it behaves exactly as GHC multiline string
• Switch fmt to always do whitespace trimming (ghc multline string are always whitespace trimmed)
• Deprecated xxxTrim functions.