preact-cli 3 and sass-loader 8

When using the latest `sass-loader` (right now it is 8.0.0) with the latest `preact-cli` (3.0.0-next.19), `preact build` fails with an error:

Module build failed (from /usr/lib/node_modules/preact-cli/node_modules/mini-css-extract-plugin/dist/loader.js):
ModuleBuildError: Module build failed (from /usr/lib/node_modules/preact-cli/lib/lib/webpack/proxy-loader.js):
ValidationError: Invalid options object. Sass Loader has been initialised using an options object that does not match the API schema.
 - options has an unknown property 'includePaths'. These properties are valid:
   object { implementation?, sassOptions?, prependData?, sourceMap?, webpackImporter? }

This happens because `includePaths` was moved to `sassOptions`. However, the issue is not yet fixed in `preact-cli`.

One of the possible solutions — use `sass-loader` 7.3.1.