Fix parsing bug in SelectorParser (#20090)

While working on another feature, I noticed that a selector such as
`.foo::before` was parsed as:

```ts
[
  {
    kind: 'compound',
    nodes: [
      { kind: 'selector', value: '.foo' },
      { kind: 'selector', value: ':' },
      { kind: 'selector', value: '::before' },
    ],
  },
]
```

Instead of:
```ts
[
  {
    kind: 'compound',
    nodes: [
      { kind: 'selector', value: '.foo' },
      { kind: 'selector', value: '::before' },
    ],
  },
]
```

So far this hasn't been a real issue in practice, but it is in a
follow-up PR that I'm working on. To keep things separated, I wanted to
fix this behavior in a dedicated PR instead.

## Test plan

1. Added a test case for this situation
2. Other tests still pass

packages/tailwindcss/src/selector-parser.test.ts

@@ -21,6 +21,18 @@ describe('parse', () => {
     ])
   })
 
+  it('should parse a pseudo-element selector with double ::', () => {
+    expect(parse('.foo::before')).toEqual([
+      {
+        kind: 'compound',
+        nodes: [
+          { kind: 'selector', value: '.foo' },
+          { kind: 'selector', value: '::before' },
+        ],
+      },
+    ])
+  })
+
   it('should parse a selector list', () => {
     expect(parse('.foo,.bar')).toEqual([
       {

packages/tailwindcss/src/selector-parser.ts

@@ -387,6 +387,11 @@ export function parse(input: string) {
       case FULL_STOP:
       case COLON:
       case NUMBER_SIGN: {
+        if (currentChar === COLON && buffer === ':') {
+          buffer += input[i]
+          break
+        }
+
         // Handle everything before the combinator as a selector and
         // start a new selector
         if (buffer.length > 0) {