Add FunctionQuery support for calling TypeDB schema functions

[CaliLuke]

Add support for calling TypeDB fun schema functions from Go.

Needs:

• FunctionCallExpr in AST
• FunctionQuery builder in gotype/query.go that generates match ... let \$result = fun_name(...); return \$result; patterns

The tqlgen parser already extracts FunctionSpec — wire it up so generated code can call these functions.

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