CI is broken on newer Node versions

[Alex]

newer versions of Node will fail until some dependencies are updated. I've been experimenting in my fork to find out which should be updated.

• the transitive dependency nan fails with

npm ERR! ../../../nan/nan_callbacks.h:55:23: error: no member named 'AccessorSignature' in namespace 'v8'
npm ERR! typedef v8::Local<v8::AccessorSignature> Sig;

See alexrudd2@b05846f where I forced a new version, but by alexrudd2@1f42bbe it was no longer necessary. I'm not sure exactly which direct dependency is bringing it in.

• webpack needs an update to v5. Note it must be updated at the same time as html-webpack-plugin since v4 and v5 are incompatible. (alexrudd2#48)
• perhaps others :)