msgid values needs to be escaped when folded

Currently only short strings get escaped properly but long strings are just split and used as is without escaping. This commit forces JSON.stringify to every line of folded msgid

**Example**

Consider the following input

```javascript
gettext('short "string"')
gettext('a long "string", ...')
```

Current PO file with invalid escaping when folding (notice the quotes around "string"):

```
msgid "short \"string\""
msgstr ""

msgid ""
"a long "string", ..."
msgstr ""
```

The same with this pull request:


```
msgid "short \"string\""
msgstr ""

msgid ""
"a long \"string\", ..."
msgstr ""
```