Update Stylelint and Postcss

R=jacktfranklin@chromium.org

Bug: none
Change-Id: I961be18530d3ccbc4efed0c2cda9bd603b14e27b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3372931
Reviewed-by: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Tim Van der Lippe <tvanderlippe@chromium.org>
diff --git a/node_modules/known-css-properties/index.d.ts b/node_modules/known-css-properties/index.d.ts
new file mode 100644
index 0000000..f31ec7b
--- /dev/null
+++ b/node_modules/known-css-properties/index.d.ts
@@ -0,0 +1,3 @@
+declare module "known-css-properties" {
+  export const all: string[];
+}