Deprecate multiline string attributes

[jcamiel]

Multilines string in Hurl can take two "attributes" escape novariable. By default, multiline string doesn't escape ("\t" is a two char) and interpolate variables.

The attributes are parsed, but I'm not sure we've implemented them (there is no integration tests that used them it if I'm not mistaken). There is no mention of these attributes on the doc https://hurl.dev also.

A "normal" multlinestring:

POST https://foo.com
```
toto tutu titi
```

escape attribute \t, \n etc... are replaced by a real tab, a newline etc....

POST https://foo.com
```escape
toto tutu titi
```

A novariable attribute:

POST https://foo.com
```novariable
toto tutu titi
```

Both attributes can be combined:

POST https://foo.com
```novariable,escape
toto tutu titi
```

And the attributes can be used on a JSON multiline string:

POST https://foo.com
POST https://example.org/api/dogs
```json,escape,novariable
{
    "id": 0,
    "name": "Frieda"
}
```

Related, there is a proposal #1733 to support "raw" multilines string (with no variable interpolation):

POST https://foo.com
```raw
toto tutu titi
```

@fabricereix @lepapareil I'm proposing to deprecate/remove both attributes escape/novariable. If we don't want variable, we can use raw string. I'm not sure if we need the really need the escape attribute.