Add minor pre-release tweaks

[tuckermcclure]

Implemented the release-hardening fixes.

Changed:

• Fixed custom leap_second_table handling throughout src/UTCDates.jl, including after, string construction, and validation of custom positive/negative leap seconds.
• Replaced public @assert validation with ArgumentError.
• Fixed iso8601(io, d; digits = 0) to match iso8601(d; digits = 0).
• Anchored the ISO parser regex and fixed the stale SimpleUTCDates error text.
• Added real tests for the previously unasserted custom-table cases in test/runtests.jl.
• Added registry compat metadata in Project.toml.
• Expanded CI to Julia 1.10 (LTS) and 1.12 in .github/workflows/test.yml.
• Cleaned up and expanded readme.md with installation, julia fences, API notes, and typo fixes.

Verification:
julia --project=. -e 'using Pkg; Pkg.test()' passes cleanly.