Add option to read stdin

There's already `-O -`. typeconv could take `-` as input, too. (It would probably imply writing to stdout as well, since filename can't be determined automatically)

### Use case

We already can do this:

```shell
npx typeconv -f ts -t jsc -O - models.ts | datamodel-codegen > models.py
```

Why not the other way around?

```py
# generate_jsonschema.py

import json
from pydantic.schema import schema
from models import *

print(json.dumps(schema([MyModel, ...]), indent=2))
```

```shell
python generate_jsonschema.py | npx typeconv -f jsc -t ts - > models.ts
```