fix(zsh): Set PROMPT just once (#2428)

No need to forcefully set the `PROMPT` variable every time the prompt is
to be shown. Just set it once, leaving the command to be evaluated every
time the prompt is to be shown, by enabling the `promptsubst` option.

Setting it once is also friendlier to users that want to experiment with
another prompt theme by temporarily setting `PROMPT` to something else.
This would currently not be possible, because the variable is always
reset before every prompt draw (precmd) and keymap change
(zle-keymap-select).

Some other updates to take better advantage of the zsh script dialect:

  * `$` is not required to read variables inside `(( ))` arithmetic
    expressions.
  * The zsh dialect to check if a variable is set is `${+var}`. Better
    than `${var+1}`, which substitutes 1 if var is set, which is
    intended for more general substitutions, not just to check if var is
    set.
  * The number of jobs can be read using the `%j` escape sequence, which
    is expanded when the `promptpercent` option is set.

Also simplified a couple of code lines by avoiding a temporary
`STARSHIP_START_TIME` variable, since we already have
`STARSHIP_CAPTURED_TIME`.

0 files changed, 0 insertions, 0 deletions