Add block-no-redundant-nesting rule

[FloEdelmann]

Fixes #679.

[kristerkari]

Small thing, but it would nice if the autofix would indent the properties correctly :)

[FloEdelmann]

I don't know if/how that's possible. Could you give me a hint?

[kristerkari]

Well, actually, the core stylelint package has removed all styling related rules, so maybe the indentation is something that we want to leave to tools like prettier instead (check https://github.com/stylelint/stylelint/blob/main/docs/migration-guide/to-15.md#deprecated-stylistic-rules)

[kristerkari]

Thanks! Looks good, just a couple of small things that I commented on :)

[kristerkari]

One probablem that I noticed when reviewing this rule is that the rule does not seem to take into account that Sass supports nesting CSS properties.

This accepted test should pass:

    {
      code: `
        .foo {
          font: {
            size: 16px;
          }
        }
      `,
      description: "Sass' nested properties"
    }

...as it compiles to:

.foo {
  font-size: 16px;
}

[kristerkari]

Thanks! Looks really good now 👍

I'll release this in the next release (currently missing only #867)