Fix function list params with single default values and improve safety of loading functions

[Efnilite]

Problem

Problem 1

local function test(arg1: texts="hey"):
    broadcast {_arg1::*}

on load:
    test()

The following code would not set {_arg1::*} to any value. Therefore, this code would return null.

Problem 2
Errors occurring during loading of functions would cause the function signature to be taken. This would disallow reloading the script to try and fix the problem, as the signature would still be in memory. This is mainly to improve reliability with addons who do not correctly handle invalid values.

Solution

Problem 1
If a parameter is keyed and there is only one default value, the code will now convert this single value to a KeyedValue, allowing the variable to be set correctly.

Problem 2
Adds a try/catch to loading ScriptFunctions. On a SkriptAPIException, the signature will be de-registered. This will allow users to attempt re-registration.

Testing Completed

Problem 1
Added test to StructFunction.

Problem 2
Manual testing.

Supporting Information

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Completes: none
Related: none