chore: rename process.contextIsolation to process.contextIsolated

[MarshallOfSound]

Discussed in api-wg --> https://electronhq.slack.com/archives/CNL6E1GJD/p1616022446002700

I've updated the notes of the original PR (#28030) and will cherry pick this commit onto the backport shortly

Notes: no-notes

[codebytere]

@MarshallOfSound what semver tag should this get?

[miniak]

@MarshallOfSound can also you fix

electron/spec-main/api-browser-window-spec.ts

Lines 4309 to 4322 in 14acf00

	it('reports process.contextIsolation', async () => {
	const iw = new BrowserWindow({
	show: false,
	webPreferences: {
	contextIsolation: true,
	preload: path.join(fixtures, 'api', 'isolated-process.js')
	}
	});
	const p = emittedOnce(ipcMain, 'context-isolation');
	iw.loadURL('about:blank');
	const [, contextIsolation] = await p;
	expect(contextIsolation).to.be.true('contextIsolation');
	});
	});

[release-clerk]

No Release Notes