chore: Use .includes and .startsWith, not .indexOf (#7936)

Easier to read than the diverse collection of `=== 0` and `!== -1` and `> -1` tests.

tests/playgrounds/screenshot.js

@@ -89,8 +89,7 @@ function workspaceToSvg_(workspace, callback, customCss) {
   const css = [].slice
     .call(document.head.querySelectorAll('style'))
     .filter(
-      (el) =>
-        /\.blocklySvg/.test(el.innerText) || el.id.indexOf('blockly-') === 0,
+      (el) => /\.blocklySvg/.test(el.innerText) || el.id.startsWith('blockly-'),
     )
     .map((el) => el.innerText)
     .join('\n');