Add `verify` feature

[ipapandinas]

I am also wondering if these feature must be released once parity supports the --verifiable argument in a stable release, since the current one, 4.0.0-rc.1, is a release candidate @vsofiya

[prxgr4mm3r]

@ipapandinas It works locally on my machine. I test it this way:

1. I create a new swanky project

   
   ➜  swanky-cli git:(feature/verifiable) ✗ ../swanky-cli/bin/run.js init ../temp_proj
   ? Which contract template should we use initially? flipper
   ? What should we name your initial contract? flipper
   ? What is your name? prxgr4mm3r
   ? What is your email? 
   ? Do you want to download Swanky Node? Yes
   ✔ Copying common template files OK
   ✔ Checking dependencies OK
   ✔ Copying contract template files OK
   ✔ Processing templates OK
   ✔ Installing dependencies OK
   ✔ Initializing git OK
   ✔ Downloading Swanky node OK
   ✔ Writing config OK
   🎉 😎 Swanky project successfully initialised! 😎 🎉

➜ swanky-cli git:(feature/verifiable) ✗ cd ../temp_proj

2. From temp_project folder run
```bash
➜  temp_proj git:(master) ✗ ../swanky-cli/bin/run.js contract compile flipper --verifiable

The contract was built in RELEASE mode.

Your contract artifacts are ready. You can find them in:
/home/prxgr4mm3r/RustroverProjects/temp_proj/target/ink/flipper

  - flipper.contract (code + metadata)
  - flipper.wasm (the contract's code)
  - flipper.json (the contract's metadata)
✔ flipper Contract compiled successfully
✔ Moving artifacts OK
✔ flipper contract's TS types Generated successfully

3. Verify compiled contract

   ➜  temp_proj git:(master) ✗ ../swanky-cli/bin/run.js contract verify flipper       
   ✔ flipper Contract verified successfully

Can you try to build a contract without swanky-cli, just with cargo-contract, and let me know if it works on your side?