<!-- Please describe your changes on the following line: -->
Implements cursor positioning in input elements (text/password) via mouse. The related issue is #10083 but is only covered partly.
This PR does **not** cover:
* positioning in textarea elements via mouse
* text selection in input/textarea elements via mouse
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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix#10083 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because I can't think of a way to test mouse interaction in the current test pipeline.
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Source-Repo: https://github.com/servo/servo
Source-Revision: 68a8e1bf2b57afe9bf6c4fb295bdc5dca099a9f6