🐛 Enable 0.x with negative margins @ v4

[k-utsumi]

I set it as below, but no negative margin was added.
$spacers: (0\.5: ($spacer * .125));

After investigating, it seems that 0 \ .5 is also evaluated as 0.
So I changed it to string comparison.

HTML example: <img class="mt-n0.5">

[mdo]

Is this needed in main for v5 as well?

[k-utsumi]

Maybe not.
I think v5 doesn't use this mixin.

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v5 codes

• https://github.com/twbs/bootstrap/blob/main/scss/_utilities.scss#L340-L382
• https://github.com/twbs/bootstrap/blob/main/scss/utilities/_api.scss#L1-L17
• Dist css: https://github.com/twbs/bootstrap/blob/main/dist/css/bootstrap.css#L6960-L6986

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NOTE: v5 dist css probably doesn't have negative margins @ default

• Provide separate built with negative margins #33988
• https://getbootstrap.com/docs/5.0/utilities/spacing/#negative-margin

[mdo]

I'm good with this. Any objections @twbs/css-review?

[ffoodd]

LGTM, never thought about such values 👌