CI to prevent PRs landing if they introduce webpack warnings

[turt2live]

We keep. regressing. flexbox. rules, alongside just causing needless noise in webpack output - let's write CI that prevents this.

[turt2live]

again: matrix-org/matrix-react-sdk#10163

[germain-gg]

In my opinion the issue is with the tool we're using.
The CSS it fixes is totally valid and owes to be transpiled by PostCSS to an output that matches our browser support. Not to generate a warning.

This could be for two reasons in my opinion (but I have not spent the time to investigate further)

• Using outdated package versions
• An issue with autoprefixer

[t3chguy]

https://www.npmjs.com/package/warnings-to-errors-webpack-plugin can accomplish this but right now would fail due to compound-web introducing warnings

I agree with @germain-gg on the exact cases exemplified earlier, but also agree with Travis we should treat warnings as errors once we fix our css build path