Use new syntax highlighting engine from zyedidia/highlight

This changes all the syntax files in the runtime directory and also
changes how syntax highlighting is done from inside micro.

runtime/syntax/keymap.yaml

@@ -0,0 +1,24 @@
+filetype: keymap
+
+detect:
+    filename: "\\.(k|key)?map$|Xmodmap$"
+
+rules:
+    - statement: "\\b(add|clear|compose|keycode|keymaps|keysym|remove|string)\\b"
+    - statement: "\\b(control|alt|shift)\\b"
+    - constant.number: "\\b[0-9]+\\b"
+    - special: "="
+    - constant.string:
+        start: "\""
+        end: "\""
+        rules:
+            - constant.specialChar: "\\\\."
+    - constant.string:
+        start: "'"
+        end: "'"
+        rules:
+            - constant.specialChar: "\\\\."
+    - comment:
+        start: "^!"
+        end: "$"
+        rules: []