Function overloading fixes and improvements

[Efnilite]

Problem

1. Fixes overloading for functions with no parameters. Previously, passing no parameters to getFunction or getSignature would match any function with the specified name. Now, passing no parameters will only match functions that also have no parameters.
2. Fixes overloading for functions with array parameters. Previously, if a function had an array as parameters, any function with the same name of the function would be matched. This would not allow for two version of, for example, clamp to exist. Now, instead of matching any function with the same name, the arguments are actually checked
3. Removes outdated documentation for FunctionIdentifier (fixes FunctionIdentifier#toString throws NPE #8031)
4. Adds more detailed error messages to the "function already exists" error (fixes Reimplement existing function type and script #8030)

Solution

1. Updates candidates to avoid returning any function when zero args are provided in the provided FunctionIdentifier.
2. First, Adds a single list value check to candidates, so functions like mean actually get matched on argument types. Second, for functions like clamp(1, 2, 3), adds a check to allow 1 to pass as a list. Third, removes outdated code from SkriptParser for parsing single list value params.
3. Updated documentation.
4. Improved error message.

Testing Completed

1. Adds a test.
2. Existing tests for functions like clamp and mean are sufficient.
3. NA
4. NA

Supporting Information

Updates the test for StructFunction with local type hints as this part is now handled by FunctionReference.

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Completes: #8030, #8031
Related: none