simplify automatic var injection

Only inject `var(…)` for CSS variables, not CSS functions that look like
variables. This simplifies it by just checking for a `(`.

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/template/codemods/automatic-var-injection.ts

@@ -2,7 +2,6 @@ import { walk, WalkAction } from '../../../../tailwindcss/src/ast'
 import { type Candidate, type Variant } from '../../../../tailwindcss/src/candidate'
 import type { Config } from '../../../../tailwindcss/src/compat/plugin-api'
 import type { DesignSystem } from '../../../../tailwindcss/src/design-system'
-import * as ValueParser from '../../../../tailwindcss/src/value-parser'
 import { printCandidate } from '../candidates'
 
 export function automaticVarInjection(
@@ -74,32 +73,16 @@ export function automaticVarInjection(
 }
 
 function injectVar(value: string): { value: string; didChange: boolean } {
-  let ast = ValueParser.parse(value)
-
   let didChange = false
-  for (let [idx, node] of ast.entries()) {
-    // Convert `--my-color` to `var(--my-color)`
-    // Except if:
-    // - It's a function like `--spacing(…)`
-    // - It's preceeded by a space, e.g.: `bg-[_--my-color]` -> `bg-[--my-color]`
-    if (
-      node.kind === 'word' &&
-      node.value.startsWith('--') &&
-      !(ast[idx - 1]?.kind === 'separator' && ast[idx - 1]?.value === ' ')
-    ) {
-      node.value = `var(${node.value})`
-      didChange = true
-    }
-
-    // Remove the space "hack" before a variable. E.g.: `bg-[_--my-color]` ->
-    // `bg-[--my-color]`
-    else if (node.kind === 'separator' && node.value === ' ') {
-      ast.splice(idx, 1)
-      didChange = true
-    }
+  if (value.startsWith('--') && !value.includes('(')) {
+    value = `var(${value})`
+    didChange = true
+  } else if (value.startsWith(' --')) {
+    value = value.slice(1)
+    didChange = true
   }
 
-  return { value: ValueParser.toCss(ast), didChange }
+  return { value, didChange }
 }
 
 function injectVarIntoVariant(designSystem: DesignSystem, variant: Variant): boolean {