fix(plugin): removeAttrs: warn without attrs

[samouss]

This is a follow-up of #1560 and should fix #1581. The PR attempt to fix an issue when the plugin is called without the attrs option. The code expects that the attrs option is present and throw an error when it's not because we end up having attrs = [undefined] (see below).

Technically this should be an error but it would be a breaking change. We can start by introducing a warning (what this PR does) and in a future major version throw an error. I added a test where input = output when attrs is not provided i.e. noop.

TypeError: Cannot read properties of undefined (reading 'includes')

      94 |         for (let pattern of attrs) {
      95 |           // if no element separators (:), assume it's attribute name, and apply to all elements *regardless of value*
    > 96 |           if (pattern.includes(elemSeparator) === false) {
         |                       ^
      97 |             pattern = ['.*', elemSeparator, pattern, elemSeparator, '.*'].join(
      98 |               ''
      99 |             );

      at Object.enter (plugins/removeAttrs.js:96:23)
      at visit (lib/xast.js:68:30)
      at visit (lib/xast.js:77:7)
      at invokePlugins (lib/svgo/plugins.js:38:11)
      at optimize (lib/svgo.js:55:13)
      at test/plugins/_index.test.js:38:28

[samouss]

The input must be equal to the output because without attrs the plugin is a noop.