feat(lexer): implement numericEntity lexer (#6)

I did this yesterday actually, I just rebased it.

Reviewed-on: #6
Reviewed-by: elland <igor@elland.me>
Co-authored-by: Léana 江 <leana.jiang+git@icloud.com>
Co-committed-by: Léana 江 <leana.jiang+git@icloud.com>

test/Spec.hs

@@ -29,6 +29,7 @@ main = hspec $ do
       it "bird tracks" birdTracks
       it "module names" modules
       it "quotes" quotes
+      it "numeric entity" numericEntity
       it "ignores nesting" ignoreNesting
 
   describe "Parser" do
@@ -152,6 +153,18 @@ space = do
                   , (1, 2, Newline)
                   ]
 
+numericEntity :: Expectation
+numericEntity = do
+  "A λ"
+    `shouldLexTo` [ (1, 1, NumericEntity 65)
+                  , (1, 6, Space)
+                  , (1, 7, NumericEntity 955) -- lambda
+                  ]
+  -- Hex
+  "e"
+    `shouldLexTo` [ (1, 1, NumericEntity 101)
+                  ]
+
 monospace :: Expectation
 monospace =
   "@mono@"