feat: enable using custom assembly by specifying chrom sizes

[sehilyi]

Fix #582

Change List

This PR enables users to use custom assembly by specifying pairs of a chromosome name and its size:

editor demo (localhost)

{
  "assembly": [["foo", 1000], ["bar", 900], ["baz", 800]],
  "tracks": [
    {
      "data": {
        "type": "json",
        "values": [
          {"c": "foo", "g": 500, "v": 1},
          {"c": "bar", "g": 450, "v": 2},
          {"c": "baz", "g": 400, "v": 3}
        ],
        "chromosomeField": "c",
        "genomicFields": ["g"]
      },
      "mark": "point",
      "color": {"value": "#0072B2"},
      "stroke": {"value": "white"},
      "strokeWidth": {"value": 2},
      "x": {"field": "g", "type": "genomic"},
      "width": 800,
      "height": 30
    }
  ]
}

bigwig example

{
  "assembly": [["NC_000962.3", 4411532]],
  "data": {
    "url": "[your-bigwig-file.bw]",
    "type": "bigwig"
  },
  "mark": "bar",
  "x": {"field": "start", "type": "genomic", "axis": "top"},
  "xe": {"field": "end", "type": "genomic"},
  "y": {"field": "value", "type": "quantitative"},
  "width": 600,
  "height": 130
}

Checklist

• Ensure the PR works with all demos on the online editor
• Unit tests added or updated
• Examples added or updated
• Documentation updated (e.g., added API functions)
• Screenshots for visual changes (e.g., new encoding support or UI change on Editor)

[manzt]

Wow, this looks great! Thanks for the quick implementation. Excited to land this feature for our users.