Provide an appropriate value for `TERM_PROGRAM` and `TERM_PROGRAM_VERSION`

[davep]

Currently, if someone was to use logic similar to textual diagnose, to diagnose what terminal their application was being run under, it would appear to be whatever terminal was used to run textual-web itself[^1]. I feel it would be a good idea to set TERM_PROGRAM to textual-web and TERM_PROGRAM_VERSION to textual-web's version (or perhaps something appropriate to the terminal served to the client side; but that could get tricker as the service expands and potentially other clients could be used).

[^1]: Testing here, I ran textual-web, served a terminal with it, and the in-browser terminal had an environment that thought it was kitty.