fix[tool]: include structs in -f interface output

[sandbubbles]

What I did

fix #4282.

How I did it

How to verify it

Commit message

the `-f interface` output was missing struct definitions; this commit
fixes the issue.

this commit also does some small code touchups in the
`build_interface_output()` function, and makes the number of newlines
between functions, events and structs consistent - to be 2 `\n`
characters.

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 46.02%. Comparing base (61f7f4e) to head (5e26a3e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/compiler/output.py	68.75%	3 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (61f7f4e) and HEAD (5e26a3e). Click for more details.

HEAD has 137 uploads less than BASE

Flag	BASE (61f7f4e)	HEAD (5e26a3e)
	138	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4294       +/-   ##
===========================================
- Coverage   91.36%   46.02%   -45.34%     
===========================================
  Files         112      112               
  Lines       15849    15858        +9     
  Branches     2685     2688        +3     
===========================================
- Hits        14480     7299     -7181     
- Misses        934     8004     +7070     
- Partials      435      555      +120     

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[pcaversaccio]

I quickly tested the PR, and I think it's incomplete. It does not list the imported (via module) structs. Example from 🐍 snekmate:

vyper -f interface src/snekmate/utils/mocks/batch_distributor_mock.vy

outputs

# Functions

@payable
@external
def distribute_ether(data: Batch):
    ...


@external
def distribute_token(token: .../snekmate/venv/Lib/site-packages/vyper/builtins/interfaces/IERC20.vyi, data: Batch):
    ...

The structs (we can debate if we only want to list Batch in this case):

# Structs

struct Transaction:
    recipient: address
    amount: uint256


struct Batch:
    txns: DynArray[Transaction, 255]

are missing in the interface for batch_distributor_mock.vy. I opened an issue here to track it.

Furthermore (not related to this PR), I would like to highlight that for the usage of interfaces as types displayed within -f interface the output is weird (see above def distribute_token(token: .../snekmate/venv/Lib/site-packages/vyper/builtins/interfaces/IERC20.vyi, data: Batch):. I opened an issue here to track it.

[cyberthirst]

for output structs we output the names of their fields although they are tuples in the abi, is this intended?

# test.vyi
struct Foo:
    x: uint256

def foo() -> Foo:
    ...

-f abi (see "name": "x",)

[
  {
    "stateMutability": "nonpayable",
    "type": "function",
    "name": "foo",
    "inputs": [],
    "outputs": [
      {
        "name": "",
        "type": "tuple",
        "components": [
          {
            "name": "x",
            "type": "uint256"
          }
        ]
      }
    ]
  }