Using @starting-style CSS at-rule crashes the parser with "property missing ':'" error

[jantimon]

When using the @starting-style CSS at-rule, which is used to define starting values for properties set on an element, the parser crashes with a property missing ':' error. This issue can be consistently reproduced using the provided sample code.

Expected Behaviour

The parser should correctly process the @starting-style at-rule without errors.

Actual Behaviour

The parser crashes and throws an error stating property missing ':'.

Reproduce Scenario (including but not limited to)

Steps to Reproduce

1. Use the @starting-style CSS at-rule in a stylesheet.
2. Attempt to parse the stylesheet with the Adobe CSS Tools parser.

Platform and Version

• Adobe CSS Tools version: 4.3.3
• Node.js 18.11.0

Sample Code that illustrates the problem

const cssTools = require("@adobe/css-tools")

cssTools.parse(`
 @starting-style {
   body {
     background: orange
   }
 }
`);

Logs taken while reproducing problem

Here is a reproduction of the issue in a Runkit environment: https://runkit.com/jantimon/6658a0b8c7c7990008b1e447

The error encountered is:

property missing ':'

[jantimon]

I created a fix in #319