* shell: fix zsh color formatting for PS1 in environments
The `colorize` function in `llnl.util.tty.color` only applies proper formatting for Bash
ANSI and for console output, but this is not what zsh expects for environment variables.
In particular, when using `zsh`, `spack env activate -p` produces a `PS1` prompt that
looks like this:
```
\[\033[0;92m\][ENVIRONMENT]\[\033[0m\]
```
For zsh the formatting should be:
```
\e[0;92m[ENVIRONMENT]\e0;m
```
- [x] Add a `zsh` option to `colorize()` to enable zsh color formatting
- [x] Add conditional to choose the right `PS1` for `zsh`, `bash`, and `sh`
- [x] Don't use color escapes for `sh`, as they don't print properly
* convert lots of += lines to triple quotes
379eeda576