Support named styles

[atlipall]

Summary

• Parse OOXML <cellStyles> elements and emit a flat namedStyles array on the workbook
• Styles that inherit from a non-default named style get a parentStyle property referencing the style by name
• All style properties are flattened directly onto the named style object (no nested style wrapper)
• Backward compatible: namedStyles is only emitted when there are custom named styles beyond the default "Normal"

Example JSF output

{
  "namedStyles": [
    { "builtinId": 0, "name": "Normal", "fontScheme": "minor", "fontSize": 12 },
    { "builtinId": 5, "name": "Percent", "fontScheme": "minor", "fontSize": 12, "numberFormat": "0%" }
  ],
  "styles": [
    { "fontScheme": "minor", "fontSize": 12 },
    { "fontScheme": "minor", "fontSize": 12, "numberFormat": "0.00E+00" },
    { "parentStyle": "Percent", "fontScheme": "minor", "fontSize": 12, "numberFormat": "0.0%" }
  ]
}

• namedStyles contains the named style definitions (flat: style properties are at the top level alongside name and builtinId)
• parentStyle on a style references a named style by name (omitted when inheriting from "Normal", the default)
• Workbooks with only a "Normal" named style do not emit namedStyles (no fixture changes for existing tests)

[borgar]

Since a renderer may be looking up styles per-cell, per-render, performance can matter here. I think it would be better to use a record for the named styles:

st1 = { parentStyle: 'Normal', fontSize: 16 };
st2 = wb.namedStyles[st1.parentStyle];

This is a lot faster and easier than having to do:

st1 = { parentStyle: 'Normal', fontSize: 16 };
st2 = wb.namedStyles.find(item => item.name === st1.parentStyle);

A renderer could obviously maintain its own internal map/record but if this is a record already, the lookups can happen instantly and there is less implementation demand placed on the client.

Alternatively, we could use numerical indexes into the list:

st1 = { parentStyle: 0, fontSize: 16 };
st2 = wb.namedStyles[st1.parentStyle];

This is the same as how cells index into the style list so it is a familiar pattern, but its less nice for humans, and we do have the names here. The names map also has the advantage of ensuring that names are unique.

So, the changes I suggest:

1. Rename the parentStyle property to extends.
2. Use an object for namedStyles, types as Record<string, NamedStyle>.

What that would look like:

namedStyles: {
  Normal: { name: 'Normal', presetId: 0, fontScheme: 'minor', fontSize: 12 },
  Percent: { name: 'Percent', builtinId: 5, fontScheme: 'minor', fontSize: 12, numberFormat: '0%' },
},
styles: [
  { fontScheme: 'minor', fontSize: 12 },
  { fontScheme: 'minor', fontSize: 12, numberFormat: '0.00E+00' },
  { extends: 'Percent', fontScheme: 'minor', fontSize: 12, numberFormat: '0.0%' },
],

I realize we're then triplicating the name (in Style, NamedStyle, and in the map, but it's important to include it because otherwise the two style types are hard to tell apart.