Rename "syntax" package to "lexers"

lexers/stxDiff.go

@@ -0,0 +1,15 @@
+//go:build stxDiff || stxAll
+
+package lexers
+
+func init() {
+	syntaxMap["diff"] = &lazySyntax{init: func() []byte {
+		return []byte(`filetype: diff
+rules:
+    - statement: "(^\\+\\+\\+.*)"
+    - statement: "(^---.*)"
+    - type: "(^@@.*)"
+    - constant.number: "(^\\+.*)"
+    - preproc: "(^-.*)"`)
+	}}
+}