Create abstract conditional matcher

src/spectator/matchers/condition_matcher.cr

@@ -0,0 +1,20 @@
+require "./matcher"
+
+module Spectator::Matchers
+  # Category of matcher that checks if an actual value satisfies a condition.
+  # Sub-types must implement `#match?`, `#message`, and `#negated_message`.
+  # Those methods accept a `ValueExpectationPartial` to work with.
+  abstract struct ConditionMatcher < Matcher
+    # Determines whether the matcher is satisfied with the value given to it.
+    # True is returned if the matcher is satisfied, false otherwise.
+    abstract def match?(partial : ValueExpectationPartial(ActualType)) : Bool forall ActualType
+
+    # Describes the condition that satisfies the matcher.
+    # This is informational and displayed to the end-user.
+    abstract def message(partial : ValueExpectationPartial(ActualType)) : String forall ActualType
+
+    # Describes the condition that won't satsify the matcher.
+    # This is informational and displayed to the end-user.
+    abstract def negated_message(partial : ValueExpectationPartial(ActualType)) : String forall ActualType
+  end
+end