## Summary of the Pull Request
Improve `ATTR_ROW::ReplaceAttrs` performance by only reserving the necessary capacity instead of resizing the new run.
That way `TextAttributeRun`s are only instantiated once instead of twice.
## PR Checklist
* [ ] Closes #xxx
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
## Detailed Description of the Pull Request / Additional comments
Performance could be further improved by directly moving `TextAttributeRun`s into the new vector, but I considered this out of scope for this PR.
## Validation Steps Performed
CPU usage when running `cacafire` is slightly reduced.
4f55568a17