parse.js fixes to allow "custom" ColorSpace formats

[sidewayss]

Two very simple fixes that come out of this discussion: #593

1. The code was finding a match for a custom parse() function, but only exiting the inner of two loops. I added a new variable: found, that the inner loop sets to true so that the outer loop knows to exit. This was not only causing unnecessary iterations of the loop, it was causing issues with custom user ColorSpaces that have formats with {type:"custom"}.
2. The addition of a single ? in this line: let color = format.parse?(env.str); so that "custom" ColorSpace formats don't require a parse property. The discussion linked above creates a custom ColorSpace with a default format that has a custom serialize() property/function, but doesn't need a parse property. The current code requires it to have a dummy parse() property/function that returns falsy. This change removes that requirement and prevents the throwing of unnecessary errors.

It is worth noting that this outside loop on ColorSpace.all is inefficient relative to its contents, even after fix 1. There is only one built-in space with custom formats: sRGB, and it's the near the end of ColorSpace.all. I'm no expert, but based on what I've seen I can't imagine any other built-in spaces doing custom parsing. As for user color spaces, I don't know if any other users have created any, and mine doesn't parse. Are there pending proposals for new CSS color strings that parse in other spaces? Is there a need for a user color space that parses a custom, non-CSS color string?

If you want to ensure that user spaces and new built-in spaces can do custom parsing, you can create a new ColorSpace.custom array containing sRGB all other registered spaces that have custom formats. You could add spaces to that array in register(). Otherwise it's just sRGB, no need to iterate over any other color spaces.

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