[Fix] Adjusted Selection Direction (#8)

Changes made: Now, when users select upward, I swapped start and end positions to match. 

Also, I noticed a bug in visual block mode, you always show the whole line instead of just the selected characters. Couldn't figure out how to fix it though, since I'm not too familiar with the Vim API :(

lua/codesnap/utils/visual.lua

@@ -9,6 +9,13 @@ function visual_utils.get_selected_text()
   else
     -- 如果选中的是多行文本，则需要分别获取每一行的文本
     local selected_text = {}
+
+    -- We switch the start and end positions if the start position is after the end position
+    -- This way we can always select from the top down
+    if start_pos[2] > end_pos[2] then
+      start_pos, end_pos = end_pos, start_pos
+    end
+
     for i = start_pos[2], end_pos[2] do
       -- 使用 vim.api.nvim_buf_get_lines() 函数获取选中的文本
       local line_text = vim.api.nvim_buf_get_lines(0, i - 1, i, false)[1]