Add support for multiline f-strings

+ Extend the parser to recognize the multiline f-strings, which the
documentation already implies will work.

The syntax is like:
```
x = 'hello'
y = 'world'

msg = f'''This is a multiline string.

Sending a message: '@x@ @y@'
'''
```

which produces:
```
This is a multiline string.

Sending a message: 'hello world'

```

+ Added some f-string tests cases to "62 string arithmetic" to exercise
the new behavior.

1 files changed, 3 insertions, 3 deletions

diff --git a/docs/markdown/Syntax.md b/docs/markdown/Syntax.md
index b3b328f..a942af0 100644
--- a/docs/markdown/Syntax.md
+++ b/docs/markdown/Syntax.md
@@ -178,7 +178,7 @@ These are raw strings that do not support the escape sequences listed
 above.  These strings can also be combined with the string formatting
 functionality via `.format()` described below.
 
-Note that multiline f-strings are not supported.
+Note that multiline f-string support was added in version 0.63.
 
 ### String index
 
@@ -211,8 +211,8 @@ As can be seen, the formatting works by replacing placeholders of type
 *(Added 0.58)*
 
 Format strings can be used as a non-positional alternative to the
-string formatting functionality described above. Note that multiline f-strings
-are not supported.
+string formatting functionality described above. Note that multiline f-string
+support was added in version 0.63.
 
 ```meson
 n = 10