Refactor xmap and fix bug

[kkew3]

Summary:

• Refactor the xmap wrapper to make it cleaner.
• Fix (partially) a bug introduced in commit 2e83b1a.

Despite fixed partially, the remaining bug should not cause significant issue. Quoted from code:

Current handling does not cover every edge case, especially the case where the cursor is at the last character of the buffer in visual line mode. I'm aware of it, and intentionally mark test/cases/xmap_ge_eol.vader.j2 as IGNORED. Generally speaking, the failed case (which involves motion ge in visual line mode) does not have severe impact to users, as ge is rarely used. Some plugins, e.g. preservim/vim-markdown, even remaps it to certain plugin function. Therefore, this might be fixed in the future, but with a relatively low priority.