Allows stack.yaml files to use !include directives

[qxjit]

This allows projects that maintain multiple stack.yaml files (e.g. for testing against multiple LTS versions) to reduce the duplication between them by using includes to share common parts.

!include may also be used in config.yaml files.

The use of !include interferes with the ability of the config set command to determine where to change a setting that has been read while parsing JSON, so thes makes config set raise error if !include appears in the file being updated.

Note: Fixes for the online documentation of the current Stack release
(https://docs.haskellstack.org/en/stable/) should target the 'stable' branch,
not the 'master' branch.

Please include the following checklist in your pull request:

• Any changes that could be relevant to users have been recorded in ChangeLog.md.
• The documentation has been updated, if necessary

Please also shortly describe how you tested your change. Bonus points for added tests!

Integration tests were added!

[OwenGraves]

I'm assuming this errors out if the include file can't be found right? Not sure if we want this as part of this pr, but I could imagine it being nice to allow an optional file to mimic something like a gitignored docker.compose.override.yaml but for a stack.yaml file, though I haven't thought through the implications much yet

A use case I could think of for this would be easily allowing our flipstone-prelude debug functions locally with the flag (if someone chooses to - I know I've wanted this enabled before for more easily testing certain things) but still always failing them on CI

[qxjit]

@OwenGraves Yes it errors out. The yaml library doesn't have an option for optional includes.