Add support for "not in" and "not_in"

[StefH]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for the "not in" and "not_in" operators and updates array initializer syntax in test cases according to the latest language updates. Key changes include

• Updating existing tests to use the new square bracket array initializer syntax.
• Adding new tests to verify the correct parsing and behavior of "not in", "not_in", and combined operators.
• Modifying the core parsing logic in ExpressionParser and Tokenizer to handle the new operators.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/System.Linq.Dynamic.Core.Tests/Parser/ExpressionParserTests.cs	Updated test array initializers and added new tests for "not in" operator behavior.
test/System.Linq.Dynamic.Core.Tests/ExpressionTests.cs	Added tests for "not_in" variants and updated assertions with the new syntax.
src/System.Linq.Dynamic.Core/Tokenizer/TextParser.cs	Introduced TokenIsIdentifier and TryGetToken to support new operator detection.
src/System.Linq.Dynamic.Core/Parser/ExpressionParser.cs	Revised operator parsing logic to support "in", "not in", and "not_in" expressions.