fix: skip hlsearch highlighting when popup is open

[VimWei]

Problem

With hlsearch on, navigating the helptoc TOC popup with J/K flickers and jumps
incorrectly. helptoc's Filter() moves the main-buffer cursor (normal! 0{lnum}zt) on each
J/K, firing CursorMoved → s:StartHL(), whose search side effects (go<byte>,
norm! n, winrestview()) conflict with the popup's redraw. Checking &buftype ==# 'popup'
does not work: the event fires in the main buffer, not the popup window.

Solution

Bail out of s:StartHL() when a popup is open, via popup_list() (guarded for Vim < 8.2):

if !v:hlsearch || mode() isnot 'n' || (exists('*popup_list') && !empty(popup_list()))
    return
endif

Fix

fixes: #55