[#539] Delay trap creation until it should be active

[stefcameron]

Fixes #539

PR Checklist

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• Issue being fixed is referenced.
• Source changes maintain:
  • Stated browser compatibility.
  • Stated React compatibility.
• Unit test coverage added/updated.
• E2E test coverage added/updated.
• Prop-types added/updated.
• Typings added/updated.
• Changes do not break SSR:
  • Careful to test typeof document/window !== 'undefined' before using it in code that gets executed on load.
• README updated (API changes, instructions, etc.).
• Changes to dependencies explained.
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[changeset-bot]

🦋 Changeset detected

Latest commit: 64ac9a5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
focus-trap-react	Minor

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[codecov]

Codecov Report

Merging #546 (64ac9a5) into master (e9f2b5b) will decrease coverage by 1.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
- Coverage   96.66%   95.12%   -1.55%     
==========================================
  Files           1        1              
  Lines         120      123       +3     
  Branches       46       48       +2     
==========================================
+ Hits          116      117       +1     
- Misses          4        6       +2     

Impacted Files	Coverage Δ
src/focus-trap-react.js	95.12% <100.00%> (-1.55%)	⬇️

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