feat(ast): add v0.1 syntax tree model

[ofabiodev]

Summary

Adds the Puff v0.1 AST model for declarations, statements, expressions, strings, variables, and source spans.

Changes

• Add structured AST nodes for files, metadata, requirements, functions, events, globals, statements, and expressions.
• Add assignable variable and access-expression contracts without embedding Minecraft command strings.
• Add comprehensive tests for node construction, declaration boundaries, syntax structure, and span propagation.

Testing

make test
make build
cd src
go vet ./...