related: #11

In ra-cubicweb, tsconfig.build.json is used only for the build and it
excludes tests while the tsconfig.json file also allows to typecheck
test files. Common configuration is located into the root tsconfig.json

ESchemas now accepts the entity types as generic type parameter.
SchemaEntityTypes is now the subset of "entities" property's keys which
are strings. This is necessary because when the keyword "keyof" is used
on a generic type it returns an union of strings, numbers and symbols.
We only want to keep strings.

We change version to 0.1.0 to make a first release on npmjs.org

Demonstrates how to create relationships to existing entities

The source maps allow the browser to display original files in the
debugger and in the console.
By default, webpack only includes source maps generated in the current
build. It doesn't include source maps located in node_modules. The
source-map-loader npm packages allow webpack to discover third-party
source maps and the `"devtool": "eval-source-map"` strategy makes
webpack expose both loaded source maps and generated source maps to the
browser.

The 8080 port will be used by the cubicweb instance.