Add test cases to verify lexer's identifier validation (#2773)

tests/typ/compiler/let.typ

@@ -32,6 +32,22 @@ Three
 #test(v2, 2)
 #test(v3, 3)
 
+---
+// Test what constitutes a valid Typst identifier.
+// Ref: false
+#let name = 1
+#test(name, 1)
+#let name_ = 1
+#test(name_, 1)
+#let name-2 = 1
+#test(name-2, 1)
+#let name_2 = 1
+#test(name_2, 1)
+#let __name = 1
+#test(__name, 1)
+#let ůñıćóðė = 1
+#test(ůñıćóðė, 1)
+
 ---
 // Test parenthesised assignments.
 // Ref: false