deps: update "xml2js"

Problem:
outdated `xml2js` dependency is pulled in by `webfont`, which hasn't
been active for 2 years.

    $ npm ls xml2js
    [email protected] /Volumes/workplace/aws-toolkit-vscode
    ├─┬ @vscode/[email protected]
    │ └── [email protected]
    ├─┬ [email protected]
    │ └── [email protected]
    ├─┬ [email protected]
    │ └── [email protected]
    ├─┬ [email protected]
    │ └── [email protected]
    └── [email protected]

Solution:
Add "overrides" directive to force the transitive "xml2js" package to
a non-vulnerable version.

Author: justinmk3

package-lock.json

@@ -4403,6 +4403,11 @@
         "xml2js": "^0.6.1",
         "yaml-cfn": "^0.3.2"
     },
+    "overrides": {
+        "webfont": {
+            "xml2js": "0.5.0"
+        }
+    },
     "prettier": {
         "printWidth": 120,
         "trailingComma": "es5",