feat: add customizable ellipsis, element prefix/suffix, and show-count for DisplaySlice

[drmingdrmer]

Changelog

feat: add customizable ellipsis, element prefix/suffix, and show-count for DisplaySlice

Three new builder methods extend DisplaySlice formatting:

• .ellipsis(s) replaces the hardcoded .. truncation indicator with any string
• .elem(prefix, suffix) wraps each displayed element (but not the ellipsis)
• .show_count() appends (N total) to the ellipsis when truncation occurs

The show_count feature only allocates a format! string when both active and
truncation is needed; the common path remains allocation-free.

Example:

vec![1,2,3,4,5,6,7].display()
    .ellipsis("...").show_count().elem("'", "'").sep(", ").braces("{", "}")
// => {'1', '2', '3', '4', ...(7 total), '7'}

feat: add customizable braces for DisplaySlice

DisplaySlice previously hardcoded [ and ] as surrounding braces.
This adds a braces() builder method to configure them, following the
same pattern as sep(). Defaults remain [ / ] for backward
compatibility.

Example:

vec![1, 2, 3].display().braces("{", "}").to_string()
// => "{1,2,3}"

vec![1, 2, 3].display().braces("<", ">").sep(", ").to_string()
// => "<1, 2, 3>"

chore: Bump ver: 0.2.4

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[xp-trumpet]

@xp-trumpet reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on drmingdrmer).