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):

• Update syntaxes:
  • Alan Transcript.sublime-syntax:
    • Add .a3t as main extension (but keep .a3log).
    • Change scope from .a3log to .a3t.
    • Update scopes and references in all package files.
  • Alan Solution.sublime-syntax:
    • Add .a3s as main extension (but keep .a3sol).
    • Change scope from .a3sol to .a3s.
    • Update scopes and references in all package files.
• Rename snippets-a3sol/ folder.
• Update documentation accordingly.

NOTE — Initially I'll add the new extensions along with the old ones (.a3sol and .a3log) for a grace period, so that old projects would still be usable in Sublime ALAN.