Fix issues in self `impl Contract` implementation

[ironcev]

#7580 fixes several issues related to self impl Contracts, but not all of them. This is the list of remaining known issues in the impl Contract implementation.

• Using pub keyword on fn and const items must emit the "Unnecessary visibility qualifier, pub is implied here." error.

• impl Contract must be allowed only in contract project types. Currently we can have, e.g.,

  library;
  impl Contract { }

  This seams to be a general issue. Having impl Abi for Contract { ... } in non-contract projects currently does not render errors.

• Clashing of the generated ABI name <normalized project name>Abi with existing names. Here we need to decide on the approach. Is <normalized project name>Abi a reserved name? What if it exists in some of the dependencies in some module?

• Support for multiple impl Contract { ... } blocks. Currently, we will get confusing compilation errors on name clashing since for each block we insert <normalized project name>Abi into the namespace. Solution for this and the previous point needs to be considered together.

• Removing Abi suffix from the generated ABI name. Having Abi suffix is against Sway conventions for naming ABIs. Note that this change is a breaking change and should come with a forc migrate migration that changes all usages of <normalized project name>Abi with <normalized project name>.