This parses 'some-property: var(--some-name)' and stores its findings
in a CustomStyleValue.
It also parses the custom properties like '--some-name: some-value' and
puts them into the StyleProperty.
We had two functions for doing mostly the same thing. Combine both
of them into String::find() and use that everywhere.
Also add some tests to cover basic behavior.
If we can't parse a property we previously a log-line was generated.
However it is in our best interest to simply ignore vendor-specific
properties (e.g -moz-something or -webkit-something) since they are not
part of the spec.
The vendor-specific-ness is determined by looking whether the property
starts with '-'.
If we want to support some vendor-specific stuff, this doesn't get in
the way since this check takes place after the parser determined that
the current property is invalid.
This cuts down the log-noise of the parser.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
