[zsh] Switch to Starship as a prompt provider

I _really_ like the aesthetics of the [Pure prompt][1] and have used it
for years. However, since switching to a Yubikey and GPG certificates
for SSH, I keep running into [a bug][2] in the interaction between
pinentry-ncurses and Pure's async functionality. It mangles all future
output for the terminal, launches a spinning pinentry-curses process,
and generally wreaks havoc.

As such, I'm attempting a switch to [Starship][3] as my prompt provider
to see if that alleviates the issue. To replicate the Pure prompt's
functionality, I [patched Starship][4] locally. I intend to push this
upstream after I use it as my daily driver on both Linux and macOS for a
while.

[1]: https://github.com/sindresorhus/pure/
[2]: sindresorhus/pure#366
[3]: https://github.com/starship/starship
[4]: michaelherold/starship@cfb20c5

config/starship.toml

@@ -0,0 +1,54 @@
+format = """
+$directory( $username$hostname)( $git_branch$git_state$git_status)( $cmd_duration)
+$character
+"""
+
+add_newline = true
+command_timeout = 500
+scan_timeout = 30
+
+[character]
+success_symbol = "[↪](blue)"
+error_symbol = "[↪](red)"
+vicmd_symbol = "[↪](purple)"
+
+[cmd_duration]
+min_time = 5_000
+format = "[$duration](yellow) "
+
+[directory]
+format = "[$path]($style)"
+style = "blue"
+truncate_to_repo = false
+truncation_length = 0
+truncation_symbol = "../"
+
+[git_branch]
+format = "([$branch]($style))"
+style = "fg:242"
+
+[git_state]
+format = "[|$state](fg:242)"
+cherry_pick = "cherry"
+rebase = "rebase-i"
+
+[git_status]
+format = "([$dirty](fg:242))([ $ahead_behind](cyan))"
+ahead = "⇣"
+behind = "⇡"
+conflicted = "*"
+dirty = "*"
+diverged = "⇣⇡"
+modified = "*"
+staged = "*"
+untracked = "*"
+
+[hostname]
+format = "[@$hostname]($style)"
+style = "fg:242"
+trim_at = ""
+
+[username]
+format = "[$user]($style)"
+style_root = "fg:242"
+style_user = "fg:242"

config/zsh/configs/prompt.zsh

@@ -2,9 +2,13 @@
 
 [ "$TERM" = "dumb" ] && return
 
-export PURE_PROMPT_SYMBOL="↪"
+if command -v starship >/dev/null && [ -z "$DISABLE_STARSHIP" ]; then
+  eval "$(starship init zsh)"
+else
+  export PURE_PROMPT_SYMBOL="↪"
 
-fpath=("$ZDOTDIR/functions/prompt" $fpath)
+  fpath=("$ZDOTDIR/functions/prompt" $fpath)
 
-autoload -U promptinit && promptinit
-prompt pure
+  autoload -U promptinit && promptinit
+  prompt pure
+fi