Add eslint-import-plugin to node_modules

Will be used to make sure no default exports are in DevTools.

The PRESUBMIT.py has been updated to skip running the formatting check
if node_modules files are affected, to workaround crbug.com/1068198.

DISABLE_THIRD_PARTY_CHECK=Add plugin to node_modules

Bug: 1068198
Change-Id: I04d4dc813daa01099f21d40edf47aaefcc0b045f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2135610
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>
diff --git a/node_modules/is-string/index.js b/node_modules/is-string/index.js
new file mode 100644
index 0000000..95b7050
--- /dev/null
+++ b/node_modules/is-string/index.js
@@ -0,0 +1,24 @@
+'use strict';
+
+var strValue = String.prototype.valueOf;
+var tryStringObject = function tryStringObject(value) {
+	try {
+		strValue.call(value);
+		return true;
+	} catch (e) {
+		return false;
+	}
+};
+var toStr = Object.prototype.toString;
+var strClass = '[object String]';
+var hasToStringTag = typeof Symbol === 'function' && typeof Symbol.toStringTag === 'symbol';
+
+module.exports = function isString(value) {
+	if (typeof value === 'string') {
+		return true;
+	}
+	if (typeof value !== 'object') {
+		return false;
+	}
+	return hasToStringTag ? tryStringObject(value) : toStr.call(value) === strClass;
+};