fix(postcss-merge-longhand): preserve hyphenated custom property case

Extends the regex that is used to locate custom properties inside a
value so that their case is preserved to also preserve the case of
custom property names that contain hyphen-minus characters.

For example, the case of the custom property in the following rule
should be preserved:

```css
border: 1px solid rgba(var(--colors-secondaryColor));
```

packages/postcss-merge-longhand/src/lib/parseWsc.js

@@ -6,7 +6,7 @@ const none = /^\s*(none|medium)(\s+none(\s+(none|currentcolor))?)?\s*$/i;
 
 /* Approximate https://drafts.csswg.org/css-values-4/#typedef-dashed-ident */
 // eslint-disable-next-line no-control-regex
-const varRE = /--(\w|[^\x00-\x7F])+/g;
+const varRE = /--(\w|-|[^\x00-\x7F])+/g;
 /** @type {(v: string) => string} */
 const toLower = (v) => {
   let match;

packages/postcss-merge-longhand/test/borders.js

@@ -1195,6 +1195,11 @@ test(
   )
 );
 
+test(
+  'Should preserve case of css custom property names with hyphens',
+  passthroughCSS('h1 { border: 1px solid rgba(var(--colors-secondaryColor)); }')
+);
+
 test(
   'Should preserve case of css custom properties example 2',
   processCSS(