Web trackpad pan

[moffatman]

Use a kind of heuristic to guess at whether an event is from mouse or trackpad.

Part of https://github.com/flutter/flutter/issues/64210

Sequence

1. https://github.com/flutter/flutter/pull/112171
2. This PR

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [x] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I signed the [CLA].
• [x] All existing and new tests are passing.