* refactor(bash): use `STARSHIP_*` for the internal variable names
The current codebase uses `dbg_trap` to save the original DEBUG trap
in bash <= 4.3. However, the variable name possibly conflicts a user
variable since it is not prefixed by `_starship` or `starship_` or
`STARSHIP_`. In this patch, we rename `dbg_trap` to
`STARSHIP_DEBUG_TRAP` following other variables of
`STARSHIP_EXIT_STATUS` and `STARSHIP_PIPE_STATUS`. We also rename the
variable `_PRESERVED_PROMPT_COMMAND` to `STARSHIP_PROMPT_COMMAND`.
* fix(bash): correctly extract DEBUG trap with spaces
* fix(bash): evaluate DEBUG trap by eval
The current code executes the saved DEBUG trap just by
$_starship_dbg_trap. This causes various problems. The content of
the variable `_starship_dbg_trap` is unexpectedly subject to the word
splitting with the pathname expansions. Also, the needed shell
expansions and quote removal are not performed. With a custom IFS,
the content of the variable will be split in an unexpected way. The
saved DEBUG trap needs to be executed by eval "$_starship_dbg_trap".
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