feat: css units in fly & blur transitions

[bfanger]

For the fly transition, the x & y are passed as numbers and are used are pixel values (px)
This limits the range of what fly can do.

This PR adds support for passing the x & y as strings that can contain CSS units:

You can create an animation that are relative:

<div class="box" in:fly={{x: '100%'}} />

'100%' is based on the width of the box.

Or based on the viewport:

<div class="modal" in:fly={{y: '50vh'}} />

Other units are also supported, we use rem a lot,
currently we are calculating the px value or accept that the animation doesn't scale.
With this PR we would be able to write:

<h1 class="title" in:fly={{y: '3rem'}} />

As a guideline for the regex I looked at css notation.
'100%' ✅
'100 %' ❌

If no unit is provided it falls back to using px. '50' is interpreted as '50px'

Closes #6050

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Tests

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[GauBen]

Please add this feature to blur too, it also expects a number of pixels.

Edit: once CSSUnitValue is available on all major browsers we should support it too. Maybe you could implement it already?

[benmccann]

Would you mind fixing the small merge conflict in this PR? We'll be attempting to go through all the outstanding PRs in the coming weeks

[vercel]

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[dummdidumm]

#6050 is the alternative to this - and I'm split on which one is better. Usability wise this PR feels better as you can input the number as you would in regular styles. The downside is the slightly increased runtime code because we need that split_css_unit function.

[bfanger]

split_css_unit does add additional runtime code, but only when importing a transition that might use it.

[dummdidumm]

Thank you!

[techniq]

@Conduitry I think this is missing from the 3.57.0 changelog