Add freeze panes to worksheet views

[flother]

Support for frozen panes (aka freeze panes) was added to the JSF format in jsfkit/types#59. This adds support for exporting freeze panes from .xlsx files to JSF-formatted JSON.

Excel allows a worksheet view to be divided into panes: two panes stacked vertically, two panes stacked horizontally, or four panes that divide the sheet into quadrants. Excel supports splitting or freezing panes, but only the latter is supported in JSF. This is deliberate, because split panes is rarely used these days and likely only kept for backwards compatibility.

Frozen panes lock top rows and/or left-most columns in place while the rest of the sheet scrolls beneath them. Dividers can't be dragged to resize panes and the frozen regions can't be scrolled. Pane sizes are measured in row and/or column offsets.

xlsx-convert will now change OOXML like this:

<sheetView tabSelected="1" workbookViewId="0">
  <pane state="frozen" xSplit="1" ySplit="2" topLeftCell="C1" activePane="bottomRight"/>
</sheetView>

To JSF like this:

{
  // WorksheetView object
  // ...
  "panes": {
    "type": "frozen",
    "columns": "1",
    "rows": "2",
    "firstVisibleCell": "C1",
    "activePane": "bottomEnd"
  }
}

This requires a new release of the jsfkit/types package with the changes from jsfkit/types#59. Until then the typecheck CI job will fail.

[flother]

This requires a new release of the jsfkit/types package with the changes from jsfkit/types#59. Until then the typecheck CI job will fail

New version released and pulled in in cc45b69.