Bug 65510

XLSB supports a set of undocumented record types (12-18) which are "short" versions of other cell records.  The short records do not specify a column index.  They assume placement adjacent to the previous cell.


Placement logic: for example, if a BrtCellBlank specifies column D and is followed by a record of type 16 (short BrtCellReal), the new cell will be in column E.


Storage: To explain the file layout, record type 13 is a short form of BrtCellRk (record type 2).  BrtCellRk is a 12 byte structure:

```
column index (4 bytes)
style index (3 bytes)
flags (1 byte)
value stored as RkNumber (4 bytes)
```

The short form is an 8 byte structure:

```
style index (3 bytes)
flags (1 byte)
value stored as RkNumber (4 bytes)
```


Documentation: Not even the 1.0 MS-XLSB spec mentions those records :( 
 https://oss.sheetjs.com/notes/xlsb_short_records/#records is a small table noting the corresponding records for those record types.

There are two sample files:

https://oss.sheetjs.com/notes/xlsb_short_records/brt_sst.xlsb includes short records for the 5 non-string record types as well as short BrtCellIsst (shared string table)

https://oss.sheetjs.com/notes/xlsb_short_records/brt_str.xlsb includes short records for the 5 non-string record types as well as short BrtCellSt (inline strings)