add Excel formula lexer

[afeld]

In order to show example Excel formulas, it would be useful to have language support for them. The specific use case is putting Excel formula examples in the page comparing spreadsheets to pandas.

This page seems to be the closes thing to a schema definition. Here is the CodeMirror file, which could be useful to…mirror 😉

Thanks!

[micahjsmith]

This would be very useful - I'd also suggest additional spreadsheet flavors like Google Sheets if possible.