createSassMigrator method to wrap common functionality

[jesstelford]

This PR belongs to a series of proposed changes:

• 👉 This PR createSassMigrator method to wrap common functionality #7499
• ⚪ Add stylelint metadata within shim #7541
• ⚪ More robust stylelint shim to support reporting #7532
• ⚪ Codify the Suggestion-on-partial-fix pattern #7543

NOTE: This PR is best viewed with whitespace changes hidden

---

The createSassMigrator utility helps encapsulate two patterns that are common across all our migrators:

1. Initialising and executing postcss
2. Ensuring the rules only get executed once.

Best visualized as a diff of what a migration might look like:

-import type {FileInfo} from 'jscodeshift';
-import postcss, {Plugin} from 'postcss';
 import valueParser from 'postcss-value-parser';
+
+import { createSassMigrator } from '../../utilities/sass';
 
-const processed = Symbol('processed');
 
-const plugin = (): Plugin => ({
-  postcssPlugin: 'replace-sass-function',
+export default createSassMigrator('replace-sass-function', () => {
-  Declaration(decl) { 
+  return ((root) => {
+    root.walkDecls((decl) => {
-    // @ts-expect-error - Skip if processed so we don't process it again
-    if (decl[processed]) return;
-
     const parsedValue = valueParser(decl.value);
 
     parsedValue.walk((node) => {
       if (!(node.type === 'function' && node.value === 'hello')) return;
 
       node.value = 'world';
     });
 
     decl.value = parsedValue.toString();
-
-    // @ts-expect-error - Mark the declaration as processed
-    decl[processed] = true;
-  },
+  });
+  };
 });
-
-export default function replaceSassFunction(fileInfo: FileInfo) {
-  return postcss(plugin()).process(fileInfo.source, {
-    syntax: require('postcss-scss'),
-  }).css;
-}

[sam-b-rose]

Nice, this is very simple an what I was imagining our Stylelint rule wrapper would look like 👍