fix: #264 convert unparsed computed style to Attr::CSSUnknown

[noahbald]

Description

This fixes a panic when using a stylesheet that has properties lightningcss is unable to parse.

Closes #264 #239

How Has This Been Tested?

• '<svg><p d="M0 0l0 1" style="fill: var(--c, #fff)"/></svg>' | oxvg optimise

Note that OXVG assumes unparseable properties are invalid, and so drops them.

Types of changes

Fixes

• TryFrom<Property> for Attr handles Property::UnparsedProperty
• get_computed_style! returns a new type ComputedStyle with Unparsed variant

Checklist:

• My code follows the code style of this project
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.