Add a deprecation warning for @import's that resolved .css files

[xzyfer]

See #2611

This is the first to removing native CSS imports.

Like Ruby Sass, Dart Sass will not implement this behaviour. This is
blocking compatibility between Node Sass and Dart Sass.

This is a non-standard behaviour which is constant source of confusion.

We had originally planned to removed this behaviour when the module
system was ready. However that has feature has been delayed and
people are continuing to be tripped up by this behaviour.

DEPRECATION WARNING on line 1, column 8 of /libsass/test.scss:
Including .css files with @import is non-standard behaviour which will be removed in future versions of LibSass.
Use a custom importer to maintain this behaviour. Check your implementations documentation on how to create a custom importer.

[xzyfer]

@nex3 feel free to word smith the deprecation warning.

Linking to a specific custom importer implementation/documentation is tricky because LibSass is mostly used embedded into other projects.

[xzyfer]

Back ported to 3.5-stable

[xzyfer]

Rebased this off of master since it only belongs in 3.5.
The deprecated raw css imports are being removed in 3.6. See #2615.

[remotezygote]

There needs to be something somewhere that tells you what to do about this warning. It cropped up all of a sudden, and I have no idea how to make it go away. Where is the documentation about what to do when you hit this? There appears to be zero documentation about sass_option_push_import_extension anywhere... did I miss something? Is that a function you call? Is that a configuration item? What is it?

[xzyfer]

There needs to be something somewhere that tells you what to do about this warning

The resolution is to stop importing .css files. This is a non-standard feature that goes against the Sass language spec.

There appears to be zero documentation about sass_option_push_import_extension anywhere

It's a temporary work around in the C API for integrators. It's use is highly discouraged! It is only offered a temporary polyfill to enable each implementation to migrate their users off of the invalid behaviour