feat: add assembler configuration (unstable)

[Julien-R44]

Added configuration for assembly as explained in this issue https://github.com/adonisjs/insiders/issues/3

Several changes compared to the original RFC:

• hooks must be defined in different files and default exported, as recommended by Virk
• assembler.runner defines the command to be executed to launch the dev server or tests. Just an object in which to define the command and its arguments

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The code that will manage the import of hook files and their application will live in adonisjs/assembler. I hesitated on this one, but after reflection I think it's the best decision

• the code will be more or less similar to this https://github.com/japa/runner/blob/develop/src/hooks.ts + we also need to import hooks file.
• This code will super specific to the assembler. It makes more sense to me that it lives in the assembler repo. A modification to it will probably also imply a modification to the assembler. So, from a maintenance point of view, it's easier to keep the two together. No release to do between adonisjs/application and adonisjs/assembler.
• It's not very important because there's not a lot of code, but this code will only be needed for development, so probably better to have it in adonisjs/assembler, which isn't installed in production.

[Julien-R44]

i have simplified the thing by removing a few hooks that won't be useful right now

also added the unstable flag. So in the adonisrc.ts file we need to use unstable_assembler: { ... }