Berry 'global.undef()' to undefine a global variable

[s-hadinger]

Description:

Berry experimental feature. Add global.undef(name: string) -> nil to undefine any global variable. Its content is not referenced by the global variable, so it may be garbage collected (if there isn't any other reference to it) and the name disappears from global scope. Solidified imports and classes are automatically reinserted to the global scope when needed.

A = 10
print(global.contains("A"))
# true

print(A)
# 10

global.undef("A")
print(A)
# BRY: Exception> 'attribute_error' - 'A' undeclared
# stack traceback:
# 	:9: in function `main`
# - this happens because the compiler knows that "A" exists at compile time, but the global does not exist at runtime

print(global.contains("A"))
# false

print(global.A)
# nil - nil is returned if the global does not exist

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.1.250203
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass