Add uuid v7 support to `std.uuid`

Add uuid v7 support to the UUID type located in `std.uuid`.
The first 48 bits of v7 stores the milliseconds since the unix epoch
(1970-01-01), additionally 74 bit are used to store random data.

Example:
---
SysTime st = DateTime(2025, 8, 19, 10, 38, 45);
UUID u = UUID(st);
SysTime o = u.v7Timestamp();
assert(o == st);

string s = u.toString();
UUID u2 = UUID(s);
SysTime o2 = u2.v7Timestamp();
assert(o2 == st);
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