[vim/vim] mapping: modifier is not recognized after a partial mapping (PR #20898)

```
Problem:  With the key protocol enabled, a mapping for a key with a modifier
          is not used when it follows a partial match of another mapping.
Solution: When the modifier was merged into the key in the typeahead, mark
          the key, so that the mapping for the simplified key can be used.
```

fixes: #12002

---

Typing "a" then CTRL-J in Insert mode inserted "a" and a line break instead
of "ax".  The cause was found by @zeertzjq in the issue: while looking for a
mapping starting with "a", the modifier of CTRL-J is merged into the key, and
the mapping for the simplified key is then skipped because the key protocol
is enabled.

That mapping is skipped so a NL typed by the user does not use a mapping for
CTRL-J.  The reason does not hold for a key that Vim simplified itself, so
those are now marked in typebuf.tb_noremap[].  The added test also checks
that a real NL still does not use the mapping.

The flag is a new bit in tb_noremap[].  All uses of that array are in
getchar.c; the one place where the value leaves the file, KeyNoremap, masks
the bit out.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20898

-- Commit Summary --

  * mapping: modifier is not recognized after a partial mapping

-- File Changes --

    M runtime/doc/todo.txt (4)
    M src/getchar.c (24)
    M src/testdir/test_termcodes.vim (24)

-- Patch Links --

https://github.com/vim/vim/pull/20898.patch
https://github.com/vim/vim/pull/20898.diff

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