[fine] Working on parser resilience

fine/tests/errors/resilience/README.md

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+These tests all produce errors in their parse, but the point is that
+the trees are kinda as best as we can get.
+
+See e.g. https://matklad.github.io/2023/05/21/resilient-ll-parsing-tutorial.html

fine/tests/errors/resilience/function_extra_comma.fine

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+fun f1(x: f64,
+
+fun f2(x: f64,, z: f64) {}
+
+fun f3() {}
+
+// @concrete:
+// | File
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f1"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"x"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error:'"Error at 'fun': expect ')' to end a parameter list"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f2"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"x"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error
+// |         Error:'"Error at ',': expected parameter"'
+// |         Comma:'","'
+// |       Parameter
+// |         Identifier:'"z"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |       RightParen:'")"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       RightBrace:'"}"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"f3"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       RightParen:'")"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       RightBrace:'"}"'
+//

fine/tests/errors/resilience/incomplete_function.fine

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+fun fib_rec(f1: f64,
+
+fun fib(n: f64) -> f64 {
+  fib_rec(1, 1, n)
+}
+
+// @concrete:
+// | File
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"fib_rec"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"f1"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |         Comma:'","'
+// |       Error:'"Error at 'fun': expect ')' to end a parameter list"'
+// |   FunctionDecl
+// |     Fun:'"fun"'
+// |     Identifier:'"fib"'
+// |     ParamList
+// |       LeftParen:'"("'
+// |       Parameter
+// |         Identifier:'"n"'
+// |         Colon:'":"'
+// |         TypeExpression
+// |           Identifier:'"f64"'
+// |       RightParen:'")"'
+// |     ReturnType
+// |       Arrow:'"->"'
+// |       TypeExpression
+// |         Identifier:'"f64"'
+// |     Block
+// |       LeftBrace:'"{"'
+// |       ExpressionStatement
+// |         CallExpression
+// |           Identifier
+// |             Identifier:'"fib_rec"'
+// |           ArgumentList
+// |             LeftParen:'"("'
+// |             Argument
+// |               LiteralExpression
+// |                 Number:'"1"'
+// |               Comma:'","'
+// |             Argument
+// |               LiteralExpression
+// |                 Number:'"1"'
+// |               Comma:'","'
+// |             Argument
+// |               Identifier
+// |                 Identifier:'"n"'
+// |             RightParen:'")"'
+// |       RightBrace:'"}"'
+//