provide a default cabal project file default through direnv

[jschaul]

Aiming to provide a sane default for everyone using direnv and compiling with cabal. Direnv shouldn't be in use by CI.

Allows writing custom options in cabal.project.local-custom; otherwise, if that file does not exist, uses the -default one.

[smatting]

The advantage of this PR over customer .envrc.local is that newcomers will get sane defaults

[pcapriotti]

Why add yet another file that we have to keep in sync, when there's already a script that is able to generate it and that needs to be used anyway to tweak it? Instead of this, we can maybe add something to .envrc that warns you when cabal.project.local is missing.

[jschaul]

Obsolete.