Add Timestamp{P} compatibility implementation

[quinnj]

This adds Timestamp{P} to Durations so packages can use the type proposed for the Julia 1.14 Dates stdlib in JuliaLang/julia#62994 before it ships.

What this does

• Durations.Timestamp is Dates.Timestamp when the Dates stdlib defines it. Durations.TIMESTAMP_FROM_DATES reports which case is active. unix2timestamp, timestamp2unix, and ISOTimestampFormat follow the same rule and are public (not exported) on Julia 1.11+.
• Otherwise src/timestamp.jl provides a copy of the stdlib implementation: Timestamp{P} for P in Second, Millisecond, Microsecond, Nanosecond; constructors, accessors, conversions, promotion, cross-type comparison, wrapping +/-, checked rounding, adjusters, ranges, parsing, and formatting. Plain Timestamp(...) is nanoseconds.
• On the compatibility path, loading Durations registers the n fractional-second format code and the Timestamp token order with Dates (at load time and again from __init__, since Dates globals are not persisted by precompilation).
• Timestamp is exported alongside Duration. Version bumped to 1.1.0.

Compatibility differences (documented in the README)

• hash agrees with == between Timestamp and DateTime and across resolutions by hashing like the equal DateTime. Date/Timestamp equal values hash differently, matching the existing Date/DateTime behavior on these Julia versions.
• n parses and formats Timestamp and formats DateTime/Time; parsing DateTime/Time with n is unreliable before 1.14 and the README says to parse a Timestamp instead.
• now(Timestamp) uses clock_gettime on POSIX and GetSystemTimePreciseAsFileTime on Windows, since older bundled libuv builds lack uv_clock_gettime.
• isoyear/isoweekdate methods exist only where Dates defines them (1.13+). Negative-year string parsing needs the 1.12 Dates parser.

Validation

• test/timestamps.jl is the stdlib test file from the PR, adapted with the gates above. It runs against whichever implementation is active.
• Passed locally: Julia 1.10.11, 1.11.9, 1.12.6 (compat path via Pkg.test), a 1.14-dev build without the PR (compat path), and the same build with the PR's Dates checkout on the load path (alias path; confirms Durations.Timestamp === Dates.Timestamp).
• The trim workload now constructs, adds, subtracts, rounds, converts, and hashes a Timestamp; it compiles and runs under --trim=safe on 1.12.6 locally.
• Not verified locally: Windows (the GetSystemTimePreciseAsFileTime path) and nightly.

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