Include variables codeSyntax member in exported json

[yinonov]

The codeSyntax property in variables helps defining alternative values which are not supported in Figma.

codeSyntax.WEB might contain a web supported color representation (e.g. LCH) which isn't supported by Figma but might add a better alternative to web pages.

codeSyntax is already included in the Figma's variable GET API so it might already make sense to be included in the extensions property?

[yinonov]

as this issue relates to color spaces especially, design-tokens/community-group#147 introduces a way to enhance color definitions in tokens. Not sure if this should be adopted at this point (as it's still a draft at the time of this writing), but maybe be considered to plan ahead?