patch 9.1.1835: completion: not possible to style popup borders globally

patch 9.1.1835: completion: not possible to style popup borders globally

Problem:  not possible to style popup borders globally
Solution: Add the 'pumborder' option (Girish Palya)

This commit introduces a new global option, 'pumborder' ('pb'), that
allows users to define borders and optional decorations for the
completion popup menu.

```
Defines a border and optional decorations for the popup menu in
completion.  The value is a comma-separated list of keywords.

Border styles (at most one):
"single"singleuse thin box-drawing characters
"double"doubleuse double-line box-drawing characters
"round"rounduse rounded corners
"ascii"asciiuse ASCII characters (-, |, +)
"custom:XXXXXXXX"
    use eight characters given after "custom:",
    in order: top, right, bottom, left,
    topleft, topright, botright, botleft

Additional flags:
"margin"marginadds one-cell spacing inside the left and right border
"shadow"shadowdraws a shadow at the right and bottom edges

Highlight groups:
|hl-PmenuBorder|hl-PmenuBorderused for the border characters
|hl-PmenuShadow|hl-PmenuShadowused for the shadow

Examples: >
  :set pumborder=single
  :set pumborder=double,margin,shadow
  :set pumborder=custom:─│─│┌┐┘└,shadow

Border styles using box-drawing characters ("single", "double",
"round") are only available when |'encoding'| is "utf-8" and
|'ambiwidth'| is "single".  "margin" requires a border style.
See also: |ins-completion-menu|.
```

fixes: https://github.com/vim/vim/pull/18441#issuecomment-3360188458
closes: #18486
closes: #17091

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