Perf: Use charCodeAt instead of string comparison (tailwindlabs#13393)

* Use charCodeAt instead of string comparison

* Rename some things

* Fix lint issues

---------

Co-authored-by: Adam Wathan <4323180+adamwathan@users.noreply.github.com>

Author: H4ad

packages/tailwindcss/css-parser.bench.ts

@@ -0,0 +1,10 @@
+import { readFileSync } from 'node:fs'
+import { bench } from 'vitest'
+import * as CSS from './src/css-parser.ts'
+
+const currentFolder = new URL('.', import.meta.url).pathname
+const cssFile = readFileSync(currentFolder + './preflight.css', 'utf-8')
+
+bench('css-parser on preflight.css', () => {
+  CSS.parse(cssFile)
+})