Add holding opt/alt for fast scrolling

[alii]

Fixes #14612

This was a feature I dearly missed from VSCode, so adding this helped me migrate to Zed without disrupting my workflow. I found that 4.0 was a nice goldilocks multiplier and felt close/the same as the speed in VSCode.

Release Notes:

• Added faster scrolling in the editor while holding opt/alt

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[alii]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[alii]

Cool, I'll find some time to add that soon. Thx!

[alii]

@SomeoneToIgnore Please let me know if the changes made look correct! I gave it a test and everything seemed to work great.

Thanks :-)

[alii]

🥳 thnx!!