Share common ghc options and other fields in `postgrest.cabal`

Problem

Right now there is a lot of duplication in postgrest.cabal between different project components. This includes common build information, ghc-options, default language extensions and more.

Duplicated flags:

https://github.com/PostgREST/postgrest/blob/ca4a6d9e9910a5503f6bb261c19017689ab821c7/postgrest.cabal#L176-L177

https://github.com/PostgREST/postgrest/blob/ca4a6d9e9910a5503f6bb261c19017689ab821c7/postgrest.cabal#L205-L207

Duplicated language extensions:

https://github.com/PostgREST/postgrest/blob/ca4a6d9e9910a5503f6bb261c19017689ab821c7/postgrest.cabal#L41-L44

https://github.com/PostgREST/postgrest/blob/ca4a6d9e9910a5503f6bb261c19017689ab821c7/postgrest.cabal#L221-L224

... and more.

Also discussed in https://github.com/PostgREST/postgrest/pull/5139#discussion_r3699864634

Solution

We can use common stanzas to group these options and reuse in different places.

It would look something like:

common common-lang-edition
  default-language:   Haskell2010

common common-exts
  default-extensions: OverloadedStrings
                      NoImplicitPrelude
                      NumericUnderscores

common common-ghc-options
  ghc-options:        -j -Werror -Wall -fwarn-identities
                      -fno-spec-constr -optP-Wno-nonportable-include-path

.
.
.

library
  import:             common-lang-edition, common-exts, common-ghc-options
  ...

executable postgrest
  import:             common-lang-edition, common-exts, common-ghc-options
  ...

test-suite spec
  import:             common-lang-edition, common-exts, common-ghc-options
  default-extensions: QuasiQuotes
  ...

Notes

We should discuss how to group these effectively, the above is just an example.

Start in postgrest.cabal at the duplicated build fields, GHC options, and language extensions linked in the issue. Read the Cabal common stanza documentation and the discussion in pull request #5139 before deciding how to group the shared settings. Done means the relevant library, executable, and test-suite sections reuse common stanzas without duplicated options.