feat: add arity getter to Fn class and rename numParams to arity

Add a public `arity` getter to the Fn class that returns the number of required arguments for a function. Internally rename the `numParams` field to `arity` across the codebase for consistency, update the C API parameter name from `numParams` to `arity` in `wrenDefineMethod` and `wrenDefineStaticMethod`, and adjust the error message in `callFunction` to use the new field name. Also update documentation to clarify that extra arguments are ignored rather than causing an error, and add a new test file `test/function/arity.wren` verifying arity values from 0 to 4.

doc/site/functions.markdown

@@ -115,8 +115,8 @@ Here we're passing a function to `greet` that takes two parameters, `first` and
       }
     }
 
-It's an error to call a function with fewer or more arguments than its
-parameter list expects.
+It's an error to call a function with fewer arguments than its parameter list
+expects. If you pass too *many* arguments, the extras are ignored.
 
 ## Returning values