Rename `value` into `syntax` in consolidated CSS file?

[tidoust]

In the CSS extracts and in the consolidated CSS file, syntaxes are stored under a value key. MDN data uses a syntax key. I had stuck to value mostly because I did not want to create an additional update burden on existing @webref/css consumers, but then I find syntax to be a better name.

As we're considering breaking changes to the consolidated CSS file (#1858), and since bumping the version of @webref/css will require code adjustments in any case, I'm wondering whether I should also change that. @dontcallmedom, WDYT?

[dontcallmedom]

+1 - improved semantics and mdn-data alignment feel quite compelling