Extend a common configuration

In monorepo scenarios it is common to have repeating task. For example running TSC in a design system mono repo. It would be beneficial to be able to extend a common configuration in such scenario.

I would like to suggest an extension API to either extend an entire configuration or single script.

To extend an entire config one could do:

```
{
scripts: {
build: 'wireit'
},
wireit: {
extends: '../config/wireit.json'
}
}
```
To extend individual scripts one would do:

```
{
scripts: {
build: 'wireit'
},
wireit: {
build: {
extends: '../config/wireit.json:build'
}
}
}
```
Note the use of `:build` similar to the syntax in dependencies.

Wireit would add the file as dependency and merge the configuration in such a way that the local config has precedence.

It was noted in Discord that the collision of `extends` at the top level of the wireit config have to be handled. I suggest a very simplistic approach where if extends is a string it is handled as an extension otherwise as a script.