Expose AbortSignal.timeout on all globals

[ptomato]

Delete the Exposed attribute from AbortSignal.timeout, letting it inherit [Exposed=*] from AbortSignal.

Cf. whatwg/html#9893 where other timers are being exposed in ShadowRealm global contexts.

• At least two implementers are interested (and none opposed):
  • T.B.D.
• Tests are written and can be reviewed and commented upon at:
  • T.B.D.
• Implementation bugs are filed:
  • Chromium: T.B.D.
  • Gecko: T.B.D.
  • WebKit: T.B.D.
  • Deno (only for aborting and events): T.B.D.
  • Node.js (only for aborting and events): T.B.D.
• MDN issue is filed: T.B.D.
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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[ptomato]

Given the discussion in tc39/proposal-shadowrealm#398, let's close this for now.