Color completion only displays color squares for hex variables

[GauravB159]

Based on issue microsoft/vscode#166197

File: cssCompletion.ts

This function only tests for built-in colors and hex values. Adding another part to it that checks for color functions fixes the two color functions.

Making this change fixes $rgb and $rgba.

[allan2]

@GauravB159 do you know if your PR also helps with vscode #166201?

[GauravB159]

@allan2 That's actually a separate issue. I've looked into that as well, and I'm mostly done with it. But there are a few tests failing. I might push a draft PR in a bit anyways because I'm not sure if those tests are based on correct behavior or not.