feat(gatsby-plugin-sass): Add ability to override sassRuleTest and sa…

…ssRuleModulesTest (#16494)

* Add a configuration for sass plugin file regexes

Add 2 new options for gatsby-sass-plugin
- `sassRuleTest` to configure regex for sass files.
- `sassRuleModulesTest` to configure regex for css modules files.

Refer #15879 for motivation
behind this addition.

* Update README

with guide to override file regex for sass and css modules.

* make string regex

* fix snapshot

packages/gatsby-plugin-sass/README.md

@@ -126,6 +126,25 @@ plugins: [
 Using CSS Modules requires no additional configuration. Simply prepend `.module` to the extension. For example: `App.scss` -> `App.module.scss`.
 Any file with the `module` extension will use CSS Modules.
 
+## SASS & CSS Modules file Regexes
+
+To override the file regex for SASS or CSS modules,
+
+```javascript
+// in gatsby-config.js
+plugins: [
+  {
+    resolve: `gatsby-plugin-sass`,
+    options: {
+      // Override the file regex for SASS
+      sassRuleTest: /\.global\.s(a|c)ss$/,
+      // Override the file regex for CSS modules
+      sassRuleModulesTest: /\.global\.s(a|c)ss$/,
+    },
+  },
+]
+```
+
 ### PostCSS plugins
 
 PostCSS is also included to handle some default optimizations like autoprefixing