allow recognition of time format in multisection numFormat

In format_code.go:248-254, it was documented that

Time formats cannot have multiple groups separated by semicolons, there is only one format. Strings are unaffected by the time format.

Apparently Excel 2021 has a default numFormat option that violate that condition: m/d/yy\ h:mm;@
It has two formatting sections. Here is a video I recorded in creating this Excel file.

This commit/PR allows recognition of time format when more than one numFormat section is present, with

first / positive section having isTimeFormat as identifier for isTimeFormat for the parsed format string
formatting for time formatted cell is taken from first/positive section's full format string.

Btw, I am coding an Excel parser that needs to cellRef.IsTime() to accurately identify date/time cells. This was the original issue that propelled me to observe and contribute.

tealeg / xlsx

allow recognition of time format in multisection numFormat #781