st_write option to abbreviate names for shapefile by truncating

[jslatane]

I know shapefiles are not a priority to support but in cases where we are forced to use them, it would be nice to have an option to abbreviate field names closer to the style of how ESRI products abbreviate, mainly by truncating. Personally I find this clearer to understand than the current style of discarding lowercase vowels, and I suspect that some other users who are forced to write into shapefiles may also be more used to the truncate method of abbreviating just due to experience with ESRI tools.

I have drafted up a function to do this along with some tests, would this be something worth creating a pull request to integrate into sf or would this be considered outside the scope of this package?