Vertical alignment of `input_dark_mode()` icon

When bringing in the dark mode switch, I noticed that the vertical alignment of the dark mode switch didn't quite line up with other icons. I thought at the time that this was a Bootstrap-specific issue, but after testing in Safari I've realized that it's likely a broader issue we should resolve (my initial testing was in Chrome).

The current state of things is that in input_dark_mode(), I disabled the vertical correction by setting the property to a space. https://github.com/rstudio/bslib/blob/b9ac2cfc0f69e08714648f8263b027fb7e9e8511/R/input-dark-mode.R#L45-L49
Users can still nudge the icon by including an inline style, e.g. style = css("--vertical-correction" = "-5px").

The Build-a-box app is a good example of the dark mode input in action.

However, with the vertical correction disabled, here's the icon appearance (on MacOS for each of these)

Chrome	Firefox	Safari

If I activate the original component rules by removing the space-toggle CSS variable from the input element, we get this:

Chrome	Firefox	Safari

(Basically they each move down by ~1px when the default vertical correction is used.)

Start in R/input-dark-mode.R at lines 45-49 and inspect how the --vertical-correction setting affects the input_dark_mode() icon. Compare the icon across Chrome, Firefox, and Safari, using the Build-a-box app as a reference; done means consistent vertical alignment without removing users’ ability to override the correction.