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):

• [x] Change file extension of all:
  • [x] Solution files: .a3sol → .a3s
  • [x] Transcripts: .a3log → .a3t
• [x] Update all repo files accordingly:
  • [x] Documentation (.md).
  • [x] Git and EditorConfig settings files.
  • [x] Scripts:
    • [x] Bash .sh
    • [x] Batch .bat
    • [x] SED scripts (no need to)