Improve the authoring experience for documenting nested properties

Multiline strings in Bicep do not allow the template author to specify which column should be considered the left-most margin. As a consequence, the `@description()` decorator expects that any indentation within a string is intentional and should be preserved.

If you're writing a description for a property on a type that's deeply nested, shunting everything over to the file's left margin can look odd:

```bicep
type myType = {
some: {
very: {
deeply: {
nested: {
@description('''A description
that
spans
multiple
lines.''')
property: string
}
}
}
}
}
```

It would be easier to follow the type's structure if Bicep multiline strings worked like C# raw string literals, e.g.:
```bicep
type myType = {
some: {
very: {
deeply: {
nested: {
@description('''A description
that
spans
multiple
lines.
''')
property: string
}
}
}
}
}
```

The description in the second example would today end with `\n `, so this would be a backwards incompatible change, though I'm not sure how disruptive it would be.

_Originally posted by @jeskew in https://github.com/Azure/bicep/issues/12412#issuecomment-1806381101_

Start by tracing how Bicep parses multiline strings and how @description() consumes them. Compare the current trailing-indentation behavior with the C# raw string example and review the backwards-compatibility impact. Done means nested multiline descriptions can preserve relative indentation without unexpectedly retaining the closing delimiter's indentation.

Written by the indexing model from the issue text.