Add tests for the indented syntax

[jamesnw]

This adds minimal coverage to the indented syntax, with focus on the areas that may change with upcoming improvements, specifically line breaks and white space parsing.

[nex3]

Please take a look at the style guide for HRX tests, in particular the sections on irrelevant placeholders and testing one thing per spec. I think a fair number of these are more complex than they need to be in an attempt to test realistic CSS, but here terse is better than realistic.

I recommend adding some error tests as well to cover cases that are invalid, especially ones that may differ in the new system.

[jamesnw]

@nex3 Do you have a preference between adding tests to a new indented-syntax folder vs. adding .sass variations of tests to files like sass-spec/spec/directives/for.hrx?

[jamesnw]

@nex3 I've made changes per the style guide, and merged in with the existing file structure. I also found https://github.com/sass/sass-spec/tree/main/spec/non_conformant/sass and wasn't sure if it would make sense to have a syntax/sass folder to hold some of these, and maybe some of the non_conformant ones?

[nex3]

You can generally ignore the non-conformant tests, although if you notice that you're writing tests that cover all the same cases go ahead and delete the redundant non-conformant tests.

[jamesnw]

Note that this behavior currently differs between the syntaxes- in a custom property, trailing whitespace is removed in Indented and condensed in Scss (see line 20).

[nex3]

There's a design conflict here between "trailing whitespace in custom property values is observable in CSS" and "end-of-line trailing whitespaces is invisible and generally shouldn't matter", complicated further by the fact that custom property trailing whitespace nearly never matters in practice. I think given that we're planning to allow --b: c ; in the indented syntax eventually anyway, preserving the existing behavior is probably the right way forward.