Fix at-rule pattern warnings to range only one line

[nlydv]

Hi, first contrib here.

I noticed the at-function-pattern and at-mixin-pattern rules spit out warnings with a range over the entire at-rule block as the "problem area", which of course doesn't make sense when it's just the name that's the issue.

Not at all familiar with the codebase and I just wanted to slap a quick fix on it if I could, best I was able to do was limiting it to one line only.

Before:

After:

[kristerkari]

@nlydv Thanks a lot! Could you please also update the unit tests to verify that the change is working?

I think that you should be able to assert the endLine and endColumn like here for example:
https://github.com/stylelint/stylelint/blob/dab8df2b3bf58e545f683a0f49d821abc089cde3/lib/rules/comment-word-disallowed-list/__tests__/index.js#L22-L25

[nlydv]

Done!

[kristerkari]

@nlydv Thanks! Great job!