Extract shared prompt values and refactor bash/git prompts

Move color constants and PS1 escape sequences from bash-prompt.sh into
a new shared rc/03-values.sh, and extract the user@host:dir fragment as
LWS_PS1_USER_HOST to eliminate duplication between bash-prompt.sh and
git-prompt.sh. Wire up git-prompt to use bash-git-prompt with the shared
prompt variables.

rc/bash-prompt.sh

@@ -2,19 +2,9 @@
 # https://www.cyberciti.biz/faq/bash-shell-change-the-color-of-my-shell-prompt-under-linux-or-unix/
 # https://linux.101hacks.com/ps1-examples/prompt-color-using-tput/
 
-RED=$(tput setaf 1)
-GREEN=$(tput setaf 2)
-CYAN=$(tput setaf 6)
-COLOR_RESET=$(tput sgr0)
-
 function get_ps1_prompt()
 {
-  local username='\u'
-  local hostname='\h'
-  local workdir='\w'
-  local user_type='\$'
-
-  echo "${debian_chroot:+($debian_chroot)}${GREEN}${username}@${hostname}${COLOR_RESET}:${CYAN}${workdir}${COLOR_RESET}${user_type} "
+  echo "${debian_chroot:+($debian_chroot)}${LWS_PS1_USER_HOST}${LWS_PS1_TYPE} "
 }
 
 function set_ps1_prompt()