patch 9.1.1512: completion: can only complete from keyword characters

patch 9.1.1512: completion: can only complete from keyword characters

Problem:  completion: can only complete from keyword characters
Solution: remove this restriction, allow completion functions when
          called from i_CTRL-N/i_CTRL-P to be triggered from non-keyword
          characters (Girish Palya)

Previously, functions specified in the `'complete'` option were
restricted to starting completion only from keyword characters (as
introduced in PR 17065). This change removes that restriction.

With this change, user-defined functions (e.g., `omnifunc`, `userfunc`)
used in `'complete'` can now initiate completion even when triggered
from non-keyword characters. This makes it easier to reuse existing
functions alongside other sources without having to consider whether the
cursor is on a keyword or non-keyword character, or worry about where
the replacement should begin (i.e., the `findstart=1` return value).

The logic for both the “collection” and “filtering” phases now fully
respects each source’s specified start column. This also extends to
fuzzy matching, making completions more predictable.

Internally, this builds on previously merged infrastructure that tracks
per-source metadata. This PR focuses on applying that metadata to
compute the leader string and insertion text appropriately for each
match.

Also, a memory corruption has been fixed in prepare_cpt_compl_funcs().

closes: #17651

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>