Adopt New Official Extensions for ALAN3 Solution and Transcript Files

[tajmone]

Switch to the upcoming official extensions .a3s and .a3t for ALAN3 solution files and transcripts, respectively (see alan-if/alan#2).

Can't even remember if the .a3sol and .a3log extensions are hardcoded into the tool, and whether it allows using custom extensions (it's been too long since I've worked on it).

Also need to check how this might affect projects which are using the ABuilder app, and ensure that they are updated in sync.