Add JSON output to stats script and use layer source

[ZeLonewolf]

Fixes #777

This PR overhauls the stats script.

First, it splits out all of the layer extraction commands into a separate "util" script since it's thematically different from stats generation.

Next, this changes the layer grouping to use the underlying source layer rather than thematic groups. This will improve our ability to assess how we're using OpenMapTiles layers.

Lastly, it adds a JSON output option, which is a prerequisite to showing stats deltas as suggested in #881.

[github-actions]

PR Preview:

• Map
• Shield Test
• style.json
• shields.json
• taginfo.json

Sprite Sheets:

[ZeLonewolf]

node scripts/stats -j -pp
Returns:

{
  "layerCount": 349,
  "styleSize": 1005817,
  "layerGroup": {
    "background": {
      "size": 122,
      "count": 1
    },
    "landuse": {
      "size": 309,
      "count": 1
    },
    "park": {
      "size": 2365,
      "count": 3
    },
    "aeroway": {
      "size": 2059,
      "count": 7
    },
    "landcover": {
      "size": 361,
      "count": 2
    },
    "boundary": {
      "size": 13691,
      "count": 9
    },
    "water": {
      "size": 912,
      "count": 3
    },
    "waterway": {
      "size": 3543,
      "count": 3
    },
    "transportation": {
      "size": 920221,
      "count": 299
    },
    "building": {
      "size": 292,
      "count": 1
    },
    "transportation_name": {
      "size": 8838,
      "count": 3
    },
    "poi": {
      "size": 10094,
      "count": 2
    },
    "water_name": {
      "size": 4647,
      "count": 2
    },
    "aerodrome_label": {
      "size": 5951,
      "count": 4
    },
    "place": {
      "size": 32062,
      "count": 9
    }
  }