Allow CSS variables in parseTheme()

[FredKSchott]

You can see the validation of colors as hex values here, where non-hex values are ignored: https://github.com/microsoft/vscode-textmate/blob/main/src/theme.ts#L126-L134

We are trying to add CSS variable support to Shiki: shikijs/shiki#212
Using a theme that uses CSS variables instead of hex values: shikijs/shiki#33 (comment)
But this issue is blocking this solution, and the end result is a color map of only #000000 and #FFFFFF.

Would you consider relaxing this check to support var(--xxxxxxxxx) as well? I'm sure this won't have much use inside of the VSCode editor, but it would have some important usage in the larger web ecosystem.

[mallardduck]

@FredKSchott - Take a look at the PR I've submitted.

I know Shiki has a workaround for now, however it's quite limited. I think it would be nice to improve that to be more versatile, hence I've created the PR here in hopes of helping move that along further.