Add built-in is_nullable() function for None/Undefined checks

[suin]

Feature Request

Is your feature request related to a problem? Please describe:
When working with KCL, I often need to check if a value is "nullish" (either None or Undefined). Currently, this requires using constructs like value in [None, Undefined] or value is None or value is Undefined. While functional, these approaches can be verbose and may lead to inconsistencies across different codebases.

Describe the feature you'd like:
I propose adding a new built-in function is_nullable(value) to KCL. This function would return True if the value is None or Undefined, and False otherwise. For example:

if is_nullable(some_value):
    print("Value is None or Undefined")
else:
    print("Value is not None and not Undefined")

This would provide a concise, readable, and standardized way to perform this common check.

Describe alternatives you've considered:

1. Continue using value in [None, Undefined]
2. Use value is None or value is Undefined

While these alternatives work, they lack the clarity and potential for optimization that a dedicated built-in function could provide.

Teachability, Documentation, Adoption, Migration Strategy:
The is_nullable function would be straightforward to teach and document. It follows a naming convention similar to other programming languages and its purpose is self-explanatory.

Adoption scenarios:

1. Simplifying null checks in conditional statements:

   if not is_nullable(config.timeout):
       # Use the timeout value

2. Cleaning up default value assignments:

   effective_value = alt_value if is_nullable(primary_value) else primary_value

3. Validating function inputs:

   assert not is_nullable(username), "Username must be provided"

Migration would be straightforward as the function doesn't break existing code. Developers could gradually adopt it in new code or during refactoring.

[suin]

'is_nullish' might actually be a better name than 'is_nullable'. It more accurately describes the function's purpose of checking for both None and Undefined.