fix(Text#fromAttributed): Fix StyleRun assuming that the startIndex and length always exist

[jonz94]

Fixes #850
Fixes #854

In AttributedText, styleRuns may not always include the startIndex or length properties

• If startIndex is missing, we assume the style applies from the beginning of the text
• If length is missing, we assume the style applies to the entire text

Example RawNode data:

{
  "contentMetadataViewModel": {
    "metadataRows": [
      {
        "metadataParts": [
          {
            "text": {
              "content": "@pbsspacetime",
              "styleRuns": [
                {
                  "weightLabel": "FONT_WEIGHT_MEDIUM",
                  "styleRunExtensions": {
                    "styleRunColorMapExtension": {
                      "colorMap": [
                        {
                          "key": "USER_INTERFACE_THEME_LIGHT",
                          "value": 4279440147
                        },
                        {
                          "key": "USER_INTERFACE_THEME_DARK",
                          "value": 4294967295
                        }
                      ]
                    }
                  }
                }
              ]
            },
            "enableTruncation": true
          }
        ]
      }
    ],
    "delimiter": "•"
  }
}