loom: implement a parser and constraint solver for semver

[aatxe]

Implements a semver library for loom, covering the two concerns needed for dependency resolution: version parsing and constraint satisfaction. Integration of this is left to follow-up work.

Parser (semver/parser.luau)

• Strict SemVer 2.0.0 parsing with full validation (leading zeros, empty identifiers, malformed pre-release/build metadata)
• Pre-release identifier comparison per the spec (numeric vs. string precedence, field-by-field)
• Constraint syntax includes: bare versions, comparison operators (>=, <=, >, <, =), caret (^), tilde (~), wildcards (*/x/X), and space-separated AND ranges
• Pre-release opt-in: pre-release versions are excluded from range matches unless the constraint explicitly names a pre-release on the same M.m.p tuple

Solver (semver/solver.luau)

• Greedy BFS dependency resolver: selects the highest satisfying version for each package, propagates transitive constraints, and retracts stale constraints when a selection changes
• Conflict detection with a human-readable error message listing all conflicting requirements and their sources
• Circular dependency detection
• Deterministic resolution order (root requirements sorted by alias)