Add ArithmeticExpr and BuiltinFunctionExpr to AST and filters

[CaliLuke]

Add to ast/nodes.go:

• ArithmeticExpr (operators: +, -, *, /, %, ^)
• BuiltinFunctionExpr (abs, ceil, floor, round, len, max, min)
• LetAssignment / MatchLetClause for computed variable bindings

Expose as filter/expression types in gotype/filter.go. These are TypeDB 3.8+ features.

Files: ast/nodes.go, ast/compiler.go, gotype/filter.go, plus unit tests