Consider merging all packages into `fuels`

[arboleya]

The idea is to experiment with merging all packages/* into packages/fuels.

A single package should simplify the repo maintenance, build times, and feedback loop.

Also, it should make it easier to make our library fully tree-shakable and avoid bloating the final build.

[arboleya]

Other points of interest:

• Review tsup dependency and configs
• Review utility of integrating with tsc project references (docs)

[petertonysmith94]

We can conclude the following discussion if we go with this approach:

• https://github.com/FuelLabs/fuels-ts/discussions/1517