add Select All to terminal context menu

The usual keyboard shortcut of Ctrl-A is not allowed since that triggers
raw REPL mode in MicroPython.

src/components/Terminal.tsx

@@ -149,6 +149,11 @@ class Terminal extends React.Component<TerminalProps> {
                     icon="clipboard"
                     label={isMacOS() ? 'Cmd-V' : 'Ctrl-V'}
                 />
+                <MenuItem
+                    onClick={() => this.xterm.selectAll()}
+                    text={i18n.translate(TerminalStringId.SelectAll)}
+                    icon="blank"
+                />
                 <MenuDivider />
                 <MenuItem
                     onClick={(): void => this.xterm.clear()}

src/components/terminal-i18n.en.json

@@ -2,6 +2,7 @@
     "terminal": {
         "copy": "Copy",
         "paste": "Paste",
+        "selectAll": "Select All",
         "clear": "Clear"
     }
 }

src/components/terminal-i18n.ts

@@ -6,5 +6,6 @@
 export enum TerminalStringId {
     Copy = 'terminal.copy',
     Paste = 'terminal.paste',
+    SelectAll = 'terminal.selectAll',
     Clear = 'terminal.clear',
 }