Add user-select: none to placeholder element

FIX: Fix an issue where some browsers wouldn't enable context menu paste when
clicking on placeholder text.

Closes codemirror/dev#1542

Author: marijnh

src/theme.ts

@@ -246,6 +246,7 @@ export const baseTheme = buildTheme("." + baseThemeID, {
     color: "#888",
     display: "inline-block",
     verticalAlign: "top",
+    userSelect: "none"
   },
 
   ".cm-highlightSpace": {