Support default initialization value for contract and struct fields? `x: u8 = 10`

### What is wrong?

It's not possible to initialize a state field upon declaration. E.g. the following does not work.

```
contract Bar:
foo: u256 = 0
```

However, the error received is very cryptic and doesn't give the user the info they need.

```
Error: at line 6 col 0, expected ENDMARKER token:
contract Bar:
^
```

### How can it be fixed

Not sure, errors are all still very rough. Just wanted to write this down to not forget about.

The issue names no files, tests, or entry points and says the desired fix is undecided. Start by tracing how contract and struct field declarations are parsed and how the shown ENDMARKER diagnostic is produced; define the supported initialization syntax and an actionable error or behavior before implementing tests.

Written by the indexing model from the issue text.