Fix flicker on document highlighting with vim 9.0.0993 (#1397)

Since v9.0.0993 vim performs a full redraw when removing text property
types, which causes flickering because vim-lsp currently removes and
re-adds the property type every time the highlight is updated. To avoid
that, we can just check if the property type already exists on the
given buffer, and only add it if it doesn't exist yet.

Author: dotdash

autoload/lsp/internal/document_highlight.vim

@@ -190,8 +190,9 @@ function! s:init_reference_highlight(buf) abort
             \   'combine': v:true,
             \   'priority': lsp#internal#textprop#priority('document_highlight')
             \ }
-        call prop_type_delete('vim-lsp-reference-highlight', l:props)
-        call prop_type_add('vim-lsp-reference-highlight', l:props)
+        if prop_type_get('vim-lsp-reference-highlight', { 'bufnr': a:buf }) == {}
+            call prop_type_add('vim-lsp-reference-highlight', l:props)
+        endif
     endif
 endfunction