Use getregion() for showing definition of selected texts

Previously, MacVim added custom VimScript to query what the user
selected text is before calling `showdefinition` on it. Since Vim has
since implemented the `getregion()` API that provides that info
natively, use that API instead of the custom implementation.

Also, the previous implementation was broken when dealing with wide
characters where it would select more characters than necessary. Using
the native `getregion()` is much more robust and handle those
situations.

runtime/autoload/macvim.vim

@@ -3,62 +3,11 @@ vim9script
 # Maintainer:   Yee Cheng Chin (macvim-dev@macvim.org)
 # Last Change:  2023-03-15
 
-# Retrieves the text under the selection, without polluting the registers.
-# This is easier if we could yank, but we don't know what the user has been
-# doing. One way we could have accomplished this was to save the register info
-# and then restore it, but this runs into problems if the unnamed register was
-# pointing to the "* register as setting and restoring the system clipboard
-# could be iffy (if there are non-text items in the clipboard). It's cleaner
-# to just use a pure Vimscript solution without having to rely on yank.
-def SelectedText(): string
-  var [line_start, column_start] = getpos("'<")[1 : 2]
-  var [line_end, column_end] = getpos("'>")[1 : 2]
-  final lines = getline(line_start, line_end)
-  if len(lines) == 0
-    return ''
-  endif
-
-  const visualmode = visualmode()
-
-  if visualmode ==# 'v'
-    if line_start == line_end && column_start == column_end
-      # Exclusive has a special case where you always select at least one
-      # char, so just handle the case here.
-      return lines[0][column_start - 1]
-    endif
-    if &selection ==# "exclusive"
-      column_end -= 1 # exclusive selection don't count the last column (usually)
-    endif
-    lines[-1] = lines[-1][ : column_end - 1]
-    lines[0] = lines[0][column_start - 1 : ]
-  elseif visualmode ==# "\<C-V>"
-    if column_end <= column_start
-      # This can happen with v_O, need to swap start/end
-      const temp = column_start
-      column_start = column_end
-      column_end = temp
-      # Also, exclusive mode is weird in this state in that we don't need to
-      # do column_end -= 1, and it acts like inclusive instead.
-    else
-      if &selection ==# "exclusive"
-        column_end -= 1 # normal exclusive behavior, need to cull the last column.
-      endif
-    endif
-    for idx in range(len(lines))
-      lines[idx] = lines[idx][column_start - 1 : column_end - 1]
-    endfor
-  else
-    # Line mode doesn't have to do anything to trim the lines
-  endif
-  return join(lines, "\n")
-enddef
-
-
 # Ask macOS to show the definition of the last selected text. Note that this
 # uses '<, and therefore has to be used in normal mode where the mark has
 # already been updated.
 export def ShowDefinitionSelected()
-  const sel_text = SelectedText()
+  const sel_text = join(getregion(getpos("'<"), getpos("'>"), { type: visualmode(), exclusive: (&selection ==# "exclusive") }), "\n")
   if len(sel_text) > 0
     const sel_start = getpos("'<")
     const sel_screenpos = win_getid()->screenpos(sel_start[1], sel_start[2])