Allow ShieldJSON to specify a map of banners on a base shield instead of requiring separate entries

[ZeLonewolf]

This PR allows a user to specify bannered routes as a variant of a base route rather than having to specify an entire route definition block for each bannered variant. This reduces the total size of the uncompressed ShieldJSON by 18kb and is one step towards refactoring it to a small enough size to consider checking in the JSON rather than code to generate it.

Currently, bannered routes are defined as distinct top-level entries in the ShieldJSON map, as follows:

[
    "US:US": {
      "spriteBlank": [
        "shield_badge_2",
        "shield_badge_3"
      ],
      "textColor": "black",
      "padding": {
        "left": 3,
        "right": 3,
        "top": 4,
        "bottom": 5
      }
    },
    "US:US:Truck": {
      "banners": [
        "TRK"
      ],
      "spriteBlank": [
        "shield_badge_2",
        "shield_badge_3"
      ],
      "textColor": "black",
      "padding": {
        "left": 3,
        "right": 3,
        "top": 4,
        "bottom": 5
      }
    },
]

This is awfully repetitive - the entire definition is repeated for the special truck route. Instead, this PR proposes that we can define a base type, and then have a list of variant networks that have all the same styles but with one or more banners attached. To do this, we define a new parameter bannerMap which contains a mapping of route network to banner list. These are unpacked at runtime into the original structure above.

The new structure for above would now look as follows:

[
    "US:US": {
      "spriteBlank": [
        "shield_badge_2",
        "shield_badge_3"
      ],
      "textColor": "black",
      "padding": {
        "left": 3,
        "right": 3,
        "top": 4,
        "bottom": 5
      }
      "bannerMap": {
        "US:US:Truck": ["TRK"],
      }
    },
]

The bannerMap can be extended for any arbitrary number of bannered variants.

This PR initially implements the bannerMap feature on the US:US network and its bannered variants as a proof of concept for feedback. If we think this makes sense, remaining work for this implementation is to flesh out the documentation and go through all the route networks that have bannered variants. I anticipate we can get significant size savings in the ShieldJSON by eliminating all the duplication from banners.

Beyond US:US, there are 117 uses of the banneredShield function in shield_defs.js:

After this PR:

[ZeLonewolf]

Compressed size difference is minimal (after all, we're removing repetition):

[claysmalley]

Sorry to leave this hanging. Looks good.