PR templates - support for inputs

Hi,

This is a request for a feature:
I'd like to have a possibility to specify a fields, that needs to be/or can be entered easily in a PR template.

For example, I want a developer to link a Jira issue in a PR it is related to (we have it in commit, ideally in each one, but it easier to have it in PR description).

So similarly to simple checkbox, I'd like to have easy way in "Preview" tab for PR authors to enter required inputs.

```markdown
Jira issue: {TODO}
```

Maybe a first version might be as simple as this: the text between `{}` will be replaced (so it will work first time only) on first edit, Maybe someone could extend this for edits.

Also I'd like to have an option to mark it mandatory, for example

```markdown
Jira issue: {*TODO}
```

might be a version for mandatory input, so only draft PR can be created if not filled in.

Thanks!

Empieza por localizar el punto de entrada para el procesamiento de PR-template en este JavaScript GitHub wrapper y determina si el comportamiento de entrada de la plantilla se expone aquí o pertenece al propio GitHub. Define cómo deben funcionar la sustitución de marcadores de posición, los campos obligatorios, la primera edición y las ediciones posteriores; para completarlo se requerirían un diseño acordado y pruebas para esos comportamientos.

Escrito por el modelo de indexación a partir del texto del issue.