Support none type

[Anandbrewery]

What issue is this change attempting to solve?

PR is dedicated to resolve None type issue faced below
Fixes #295

How does this change solve the problem? Why is this the best approach?

• EtNil constant is introduced in interface.go
• By intercepting "None" (case-insensitive) in parser.go and assigning it EtNil, we provide a type-safe way for functions to handle null arguments. This follows the existing architecture for constants and strings.

How can we be sure this works as expected?

• Added unit tests in parser_test.go for standalone None and nested function calls.
• All 46 TestParseExpr cases passed on running-> (go test -v ./pkg/parser/... -run TestParseExpr)

[Anandbrewery]

hi @deniszh @grzkv @azhiltsov @timtofan @emadolsky should i submit this PR with single commit or multi commit for each change is fine.

[Anandbrewery]

hi @deniszh @grzkv could please let me know is there any issue with this PR