Follow-up of https://github.com/zed-industries/zed/pull/24857

Based on the feedback,

* made non-persisting font size change as a default in Zed keymaps
JetBrains IDEs seem to persist font size changes by default, hence left
to do so in Zed keymaps too

* fixed a bug with holding a binding to change the font size caused
flickering

Release Notes:

- N/A