add fallback fonts if Fira Code isn't available

ChromeOS-Terminal.md

@@ -9,7 +9,7 @@ term_.prefs_.set('user-css-text', `@font-face{\n    font-family: 'Fira Code';\n
 - Note, there is a bug where you need to push `ctrl-0` to get the cursor to input text at the right place.  The following settings fix that bug, but the cursor is still in the wrong place:
 ```
 @font-face{
-    font-family: 'Fira Code';
+    font-family: "Fira Code", "PT Mono", "DejaVu Sans Mono", "Noto Sans Mono", "Everson Mono", FreeMono, Menlo, Terminal, monospace;
     src: url('https://raw.githubusercontent.com/tonsky/FiraCode/5.2/distr/woff2/FiraCode-Regular.woff2') format('woff2'),
          url('https://raw.githubusercontent.com/tonsky/FiraCode/5.2/distr/woff/FiraCode-Regular.woff') format('woff'),
          url('https://raw.githubusercontent.com/tonsky/FiraCode/5.2/distr/ttf/FiraCode-Regular.ttf') format('truetype');