Add support for multi-line string literals when converting string concatenation to string interpolation

### Description

#1551 added a code action for converting string concatenation to string interpolation, however multi-line string literals are not yet supported. We should add support for this, e.g.
```swift
"""
I
am
a
""" + value + 
"""
multi-line
string
literal
"""
```
will be converted to
```swift
"""
I
am
a\(value)multi-line
string
literal
"""
```