compiler, runtime: Support Formatted Error Messages

Make it possible to output formatted error messages using a
flexible API that includes an ability to specify styles and
formatters.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/Common/Protocols.swift

@@ -71,7 +71,7 @@ public protocol ProgramExecutionEvaluator {
 }
 
 public protocol Errorable: CustomStringConvertible {
-  func format() -> String
+  func format(_ formatter: Formattable) -> String
   func msg() -> String
   func append(error: any Errorable) -> any Errorable
   func eq(_ rhs: any Errorable) -> Bool
@@ -83,6 +83,10 @@ extension Errorable {
   }
 }
 
+public protocol Formattable {
+  func formatWithStyle(_ value: String, _ style: Style) -> String
+}
+
 extension ProgramExecutionEvaluator {
   public func ExecuteStatements(
     _ statements: [EvaluatableStatement], inExecution execution: ProgramExecution