Rename ITheme.selection to selectionBackground

This makes it consistent with selectionForeground and other theme keys and
as a result will make the API easier to work with, especially when inactive
selection is done (#3803)

addons/xterm-addon-webgl/test/WebglRenderer.api.ts

@@ -843,7 +843,7 @@ describe('WebGL Renderer Integration Tests', async () => {
       const theme: ITheme = {
         foreground: '#FF0000',
         background: '#00FF00',
-        selection: '#0000FF'
+        selectionBackground: '#0000FF'
       };
       await page.evaluate(`window.term.options.theme = ${JSON.stringify(theme)};`);
       await writeSync(page, ` █\\x1b[7m█\\x1b[0m`);