[css-background-4] Replace <position> by <bg-position>

[cdoublev]

In CSS Backgrounds 4, background-position is defined with <position># and <position> is defined with a value that accepts the 3-values form, which has been removed from the definition of <position> in CSS Values 4, as explained in #2140. I believe this is the reason why CSS Backgrounds 3 defines background-position with <bg-position># instead of <position>#.

CSS Backgrounds 4 is flagged as Not Ready For Implementation but is listed in w3c/browser-specs and used by @webref/css, which picks the definition of background-position from CSS Backgrounds 4 instead of CSS Backgrounds 3 during its curation step, resulting to duplicate definitions for <position>. Do you think CSS Backgrounds 4 should not be listed in w3c/browser-specs? Otherwise, can <position> be replaced to <bg-position> in this spec, please?

FYI, @tidoust.

[fantasai]

Done