[css-images-4] Fix basic syntax of gradients

[cdoublev]

Fixes #7986.

The current syntax makes the color stop list optional, which is obviously unexpected.

This PR also aligns CSS Images 4 with CSS Images 3 regarding <size> renamed to <rg-size> (resolving a conflict with <ray()>) and <ending-shape> renamed to <rg-ending-shape> (for consistency, I guess).

I am not sure if a? b? || c is different than [a? b?]! || c so I prefered the first option for simplicity.

[tabatkins]

I am not sure if a? b? || c is different than [a? b?]! || c so I prefered the first option for simplicity.

They're not equivalent (the first is allowed to produce nothing, the second isn't), but for the purpose of this spec what you did was correct.

[cdoublev]

Right, and actually I would have preferred the other syntax for two reasons.

First, the trailing ? in (conic gradient parameters) [[[from <angle>]? [at <position>]?] || <color-interpolation-method>]? and (radial gradient parameters) [[[<radial-shape> || <radial-size>]? [at <position>]?] || <color-interpolation-method>]? is useless, which does not seem right to me.

Second, the result is not consistent with the result from parsing <linear-gradient()>.

conic-gradient(red, cyan), linear-gradient(red, cyan), radial-gradient(red, cyan), produce the following pseudo-representations:

ConicGradient(
  List(
    List(
      Omitted('from <angle>'),
      Omitted('at <position>'),
    ),
    Omitted('<color-interpolation-method>'),
  ),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)
LinearGradient(
  Omitted('[<angle> | to <side-or-corner>] || <color-interpolation-method>'),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)
RadialGradient(
  List(
    List(
      Omitted('<radial-shape> || <radial-size>'),
      Omitted('at <position>'),
    ),
    Omitted('<color-interpolation-method>'),
  ),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)

I would prefer to get this:

ConicGradient(
  Omitted('[[from <angle>]? [at <position>]?]! || <color-interpolation-method>'),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)
LinearGradient(
  Omitted('[<angle> | to <side-or-corner>] || <color-interpolation-method>'),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)
RadialGradient(
  Omitted('[[<radial-shape> || <radial-size>]? [at <position>]?]! || <color-interpolation-method>'),
  ColorStopList(Keyword('red'), Keyword('cyan')),
)