Distinguish between simple and complex initial values in data/css/properties.json

In data/css/properties.json, the initial value can be:

1. A simple value, like 0, 0% 0%, none, or auto. This used to be wrapped like <code>0</code>
2. A key into l10n/css.json, like noneButOverriddenInUserAgentCSS or noPracticalInitialValue.
3. A list of strings, which are each one of the above. In practice, all current lists appear to be lists of literals.

I've proposed https://github.com/mozilla/kumascript/pull/162 to distinguish between cases 1 and 2, but it would be better to have a clearer signal in the data. Some straw man suggestions:

Different Keys for literal vs. prose initial values. Advantage - unambiguous. Disadvantage - users must update.

"align-self": {
    "initialLiteral": "auto"
},
"all": {
    "initialProse": "noPracticalInitialValue"
},
"animation": {
   "initialLiteral": [
      "animation-name",
      "animation-duration",
      "animation-timing-function",
      "animation-delay",
      "animation-iteration-count",
      "animation-direction",
      "animation-fill-mode",
      "animation-play-state"
    ]
}

Prefix for prose value, none for literals. Advantage - same key, quick check for literal vs prose. Disadvantage - pick a universal prefix, users must process to get prose key.

"align-self": {
    "initial": "auto"
},
"all": {
    "initial": "_prose_:noPracticalInitialValue"
},
"animation": {
   "initial": [
      "animation-name",
      "animation-duration",
      "animation-timing-function",
      "animation-delay",
      "animation-iteration-count",
      "animation-direction",
      "animation-fill-mode",
      "animation-play-state"
    ]
}

Initial objects instead of strings. Advantage - unambiguous, allows for future expansion, mixing literals and prose. Disadvantage - detect object vs list, overly wordy.

"align-self": {
    "initial": {
       "value": "auto",
       "type": "literal"
   }
},
"all": {
    "initial": {
       "value": "noPracticalInitialValue",
       "type": "prose"
   }
},
"animation": {
   "initial": [
          {"value": "animation-name", "type": "literal"},,
          {"value": "animation-duration", "type": "literal"},,
          {"value": "animation-timing-function", "type": "literal"},,
          {"value": "animation-delay", "type": "literal"},,
          {"value": "animation-iteration-count", "type": "literal"},,
          {"value": "animation-direction", "type": "literal"},,
          {"value": "animation-fill-mode", "type": "literal"},,
          {"value": "animation-play-state", "type": "literal"},
    ]
}

Start by reviewing data/css/properties.json and the related l10n/css.json values, then compare the proposed handling in kumascript#162. Decide on a representation that unambiguously distinguishes literal and prose initial values, including lists, and verify that the documented data shape gives consumers enough information to process each case.