Set WISH_RIGHT_PLUGINS to be displayed on right.
wish_append is stateful now. Appends to WISH_RIGHT_PS1 or WISH_LEFT_PS1
based on WISH_STATE. wish_append also calculates the length of right
prompt. This has the downside that right prompt can't include any text
that expands on PS1. Therefore, a limitation needs to be applied on
plugins to only append text that won't be expanded in PS1. (i.e. no \u,
\h, \w, \W, etc).