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, 8 insertions, 1 deletions

diff --git a/mesonbuild/interpreterbase/interpreterbase.py b/mesonbuild/interpreterbase/interpreterbase.py
index 85aabd1..f9a3b07 100644
--- a/mesonbuild/interpreterbase/interpreterbase.py
+++ b/mesonbuild/interpreterbase/interpreterbase.py
@@ -217,7 +217,10 @@ class InterpreterBase:
         elif isinstance(cur, mparser.TernaryNode):
             return self.evaluate_ternary(cur)
         elif isinstance(cur, mparser.FormatStringNode):
-            return self.evaluate_fstring(cur)
+            if isinstance(cur, mparser.MultilineFormatStringNode):
+                return self.evaluate_multiline_fstring(cur)
+            else:
+                return self.evaluate_fstring(cur)
         elif isinstance(cur, mparser.ContinueNode):
             raise ContinueRequest()
         elif isinstance(cur, mparser.BreakNode):
@@ -367,6 +370,10 @@ class InterpreterBase:
         else:
             return self.evaluate_statement(node.falseblock)
 
+    @FeatureNew('multiline format strings', '0.63.0')
+    def evaluate_multiline_fstring(self, node: mparser.MultilineFormatStringNode) -> InterpreterObject:
+        return self.evaluate_fstring(node)
+
     @FeatureNew('format strings', '0.58.0')
     def evaluate_fstring(self, node: mparser.FormatStringNode) -> InterpreterObject:
         assert isinstance(node, mparser.FormatStringNode)