Add tests for objects with fields and nullable fields (#13)

Refine interface of Render (#8)

This pull request refactors the `Render` method of the `Template` class
in `minijinja/Environment.cs` to provide more type-safe and flexible
overloads for rendering templates with different types of context
objects. It replaces the previous single method that accepted a generic
`object?` with overloads for `IDictionary<string, Value>`,
`IDictionary<string, object?>`, and `ITemplateSerializable`. This change
simplifies usage and improves clarity in both the implementation and the
tests. All test cases are updated to use the new overload for rendering
templates without context.

### Template rendering API refactor

* Refactored the `Template.Render` method to provide overloads for
`IDictionary<string, Value>`, `IDictionary<string, object?>`, and
`ITemplateSerializable`, replacing the previous single method that
accepted a generic `object?` and a helper for context conversion. This
improves type safety and clarity for users.
[[1]](diffhunk://#diff-62578f15123609361799a7fdf8794305d7cd2dcc0c6fa6a58f48cba2ee826c74L226-R233)
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### Test updates

* Updated all test cases to use the new `Render()` overload when no
context is provided, replacing calls to `Render(null)` for clarity and
consistency with the new API.
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### Minor code cleanup

* Added a missing `using System;` directive at the top of
`minijinja/Environment.cs` for completeness.

Supports Trimming (#7)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Fix ci (#5)