Before #31525 the prompts had a trailing space.
Also add a test for #7857.

Problem:  no sanitize check when running linematch
Solution: add sanitize check before applying the linematch algorithm,
          similar to diff_find_change() (Jonathon)

closes: vim/vim#16446

https://github.com/vim/vim/commit/ca307efe486670b76563a4a287bc94dace57fb74

Co-authored-by: Jonathon <jonathonwhite@protonmail.com>

Problem:
When an evaluation {-item calls `nvim_eval_statusline()`, that nested
call may overwrite the same memory used for `stl_items`.

Solution:
Make `curitem` static and use it to compute an offset to avoid
overwriting `stl_items` in nested calls to `build_stl_str_hl()`. Move
miscellaneous statusline tests into `describe()` block.

Problem:
`nvim_win_set_config` cannot set the title and footer independently.
When only one is given, the other is reset to the default of "left".

Solution:
Reuse existing title/footer value if not provided.

Problem: Right aligned virtual text can cover up buffer text if virtual
text is too long

Solution: An additional option for `virt_text_pos` called
`eol_right_align` has been added to truncate virtual text if it would
have otherwise covered up buffer text. This ensures the virtual text
extends no further left than EOL.

Problem:  A right-click on the 'statuscolumn' does not open the
          popupmenu, even if a cell without a clickdef is clicked.
          Clicking the %C fold item does not open/close the fold.
Solution: Open the popupmenu when there is no clickdef like right-clicking
          the sign/numbercolumn does. Fill "linebuf_vcol" when drawing the
          'statuscolumn' to handle foldcolumn item clicks.

Problem:  Combined highlighting was not applied to nvim_eval_statusline(),
          and 'statuscolumn' sign segment/numhl highlights.
Solution: Add an additional `groups` element to the return value of
          `nvim_eval_statusline()->highlights`. This is an array of stacked
          highlight groups (highest priority last). Also resolve combined
          highlights for the 'statuscolumn' sign segment/numhl highlights.
          Expose/synchronize some drawline.c logic that is now mimicked in
          three different places.