Add LexAndYaccCalculator example

[brenoafb]

Reading the docs, I felt it was appropriate to have one more example that shows
how to interact with constructors in a common use case: AST parsing.

This PR adds a minimal calculator example.

To run the example, go to the tests/LexAndYaccCalculator directory and run dotnet run.

[teo-tsirpanis]

Can you remove start and use Expr as the starting symbol?

Suggested change

	%type < Syntax.Expr > start
	%%
	start: Expr { $1 }
	Expr: ID { Var($1) }
	%type < Syntax.Expr > Expr
	%%
	Expr: ID { Var($1) }