std.Progress: Terminate progress escape codes with `ST` not `BEL`

`ST` is the "string terminator" and what is actually prescribed in the
standard for escape codes. Use of `BEL` as a terminator is apparently a
historical oddity that persists for compatibility. Unfortunately, not
all terminals support using `BEL`, including Ubuntu's new default
terminal, Ptyxis. Using `ST` should make it work in more terminals.

Further reading:

- https://en.wikipedia.org/wiki/ANSI_escape_code#Operating_System_Command_sequences
- https://ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf