Add null keyword as alias for nil

[Copilot]

Adds null as a keyword that behaves identically to nil, returning NilValue.Instance.

Changes

• FluidParser.cs: Added NilKeyword static field and updated Primary parser to handle both nil and null keywords
• ParserTests.cs: Added 15 test cases verifying null has identical semantics to nil

Example

{% if x == null %}x is null{% endif %}
{% if null == nil %}both keywords equivalent{% endif %}
{{ null }}  {# renders as empty string #}

The null keyword now works in all contexts where nil is valid: comparisons, conditionals, and output expressions.

Original prompt

Add support for the null keyword. It should behave the same way as nil by returning the NilFluidValue.Instance.

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