Improve listenTo() and fire() function execution time

[mlewand]

📝 Provide detailed reproduction steps (if any)

During the initialization of an editor with short content, listenTo and fire both combined account for ~9% self execution time of the entire initialization time (~110ms out of 1.23s).

We should try to improve this.

Here is a devtools profiler dump that can be viewed later on: editor-init-small.zip.

✔️ Expected result

Faster 🏃‍♀️

❌ Actual result

Slow 🐢

📃 Other details

This is a follow-up of #5880.

• Browser: Chrome
• OS: Windows (should be cross system)
• CKEditor version: latest master

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[ma2ciek]

Taking the occasion we could check if we can get rid of symbols in the EmitterMixin, this makes our CKEditor 5 utils impossible to use together with the bundled editor.

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[CKEditorBot]

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