Editorial: use map/exists instead of "defined", "exists" etc.

[zolkis]

Over all the spec, use the [=map/exists=] construct, see https://infra.spec.whatwg.org/#map-exists

[relu91]

Call 22/05/2023:

• @zolkis should be fixed, we can close it next time once we have verified that we fixed it.

[relu91]

I verified that the construct is used in the text. I propose closing and opening concrete issues if somebody spots a violation of this rule.

[relu91]

Call 11/12:

• @zolkis still 3-4 places where it still needs to be fixed. I'd keep it open. ( to fix it, you can just look for exists keyword).