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, 22 insertions, 0 deletions

diff --git a/docs/markdown/snippets/support-multiline-fstring.md b/docs/markdown/snippets/support-multiline-fstring.md
new file mode 100644
index 0000000..296a04e
--- /dev/null
+++ b/docs/markdown/snippets/support-multiline-fstring.md
@@ -0,0 +1,22 @@
+## Added support for multiline fstrings
+
+Added support for multiline f-strings which use the same syntax as f-strings
+for string substition.
+
+```meson
+x = 'hello'
+y = 'world'
+
+msg = f'''Sending a message...
+"@x@ @y@"
+'''
+```
+
+which produces:
+
+```
+Sending a message....
+
+"hello world"
+
+```