feat: add display_debug() for formatting Option<T> via Debug

[drmingdrmer]

Changelog

feat: add display_debug() for formatting Option<T> via Debug

DisplayOption now stores a format function pointer (fmt_fn) instead of
requiring T: Display on the struct. This lets the same wrapper support both
Display and Debug formatting, selected at construction time.

DisplayOptionExt::display() sets fmt_fn to Display::fmt (unchanged
behavior). The new DisplayDebugOptionExt::display_debug() sets it to
Debug::fmt, useful for types that implement Debug but not Display
(e.g., Vec<T>).

Changes:

• Replace tuple struct with named fields (inner, fmt_fn)
• Remove T: Display bound from DisplayOption struct and its Display impl
• Add DisplayDebugOptionExt trait with .display_debug() method

chore: bump ver 0.2.1

chore: prepare crate for crates.io publication

Add complete crates.io metadata to Cargo.toml including repository,
homepage, documentation URLs, readme field, keywords, and categories
to ensure proper discoverability on crates.io.

Update README.md with corrected trait names in examples
(DisplayOptionExt, DisplayResultExt, DisplaySliceExt,
DisplayUnixTimeStampExt), add missing DisplayResult example, and
update version from 0.1.0 to 0.2.0. Remove unnecessary sections for
improved clarity.

Enhance crate-level documentation in lib.rs with comprehensive
examples for all four extension traits (Option, Result, Slice, Unix
Timestamp) to provide better API documentation on docs.rs.

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[xp-trumpet]

@xp-trumpet reviewed 4 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @drmingdrmer).