Spacing Units: Tweak step values

[stacimc]

Description

In #36186 it was noticed that some of the spacing units, in particular em and rem, are configured with a very small step value. This means that when you hold the 'up' or 'down' key to change spacing, em moves very slowly compared to px for example.

Here's a visual using the height of the Spacer block:

px-vs-em-step.mov

It's perhaps more noticeable on mobile, because the UnitControl has its own increment/decrement buttons:

The step value comes from the configuration here and was introduced in #32692.

Step-by-step reproduction instructions

1. In the block editor, insert a block that supports some spacing value. Cover is a good example, with its minHeight attribute.
2. Adjust the value of the minHeight in the Inspector Controls, by clicking into the input and holding the up or down arrow key. On mobile use the up/down buttons.
3. Try this with several different units, and notice that the em and rem increment is noticeably smaller/slower.

Screenshots, screen recording, code snippet

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Environment info

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Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[stacimc]

For a conflicting perspective, quoting @aaronrobertshaw on #36186

Another perspective to consider is that most using the step functionality will be doing so to fine-tune a value rather than set it for these units. The small step value makes more sense in this case.