Add experimental Tailwind-styled xlsx creation (bb-excel.styled)

New namespace bb-excel.styled provides a fun way to create styled Excel
spreadsheets using Tailwind CSS-like class syntax:

  (styled/create-styled-xlsx "output.xlsx"
    [[:A1.bg-blue-500.text-white.font-bold "Header"]
     [:A2.border "Data"]
     [:3.bg-gray-100 ["Row" "of" "data"]]
     [:Sheet2/A1:C3.border [[1 2 3] [4 5 6] [7 8 9]]]])

Supported features:
- Background colors: bg-{color}-{shade} (full Tailwind palette)
- Text colors: text-{color}-{shade}
- Font styles: font-bold, italic, underline
- Text sizes: text-xs, text-sm, text-base, text-lg, text-xl, text-2xl
- Alignment: text-left, text-center, text-right
- Borders: border, border-t/b/l/r, border-{color}-{shade}
- Border styles: border-dashed, border-dotted, border-double

Selectors support:
- Single cells: :A1.classes
- Rows: :5.classes [values...]
- Columns: :AA.classes {1 val1 2 val2}
- Ranges: :A1:C3.classes [[row1] [row2] [row3]]
- Sheets: :SheetName/A1.classes

This is experimental and obviously can't do hover states or transitions ;)

Author: Kay Bosompem