Move virtualtext handling to own file

This allows cursor and virtualtext to be independently autoloaded.

autoload/ale/util.vim

@@ -452,3 +452,14 @@ function! ale#util#Col(str, chr) abort
 
     return strlen(join(split(a:str, '\zs')[0:a:chr - 2], '')) + 1
 endfunction
+
+function! ale#util#FindItemAtCursor(buffer) abort
+    let l:info = get(g:ale_buffer_info, a:buffer, {})
+    let l:loclist = get(l:info, 'loclist', [])
+    let l:pos = getcurpos()
+    let l:index = ale#util#BinarySearch(l:loclist, a:buffer, l:pos[1], l:pos[2])
+    let l:loc = l:index >= 0 ? l:loclist[l:index] : {}
+
+    return [l:info, l:loc]
+endfunction
+