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, 7 insertions, 0 deletions

diff --git a/test cases/common/62 string arithmetic/test.json b/test cases/common/62 string arithmetic/test.json
new file mode 100644
index 000000000..e3519d050
--- /dev/null
+++ b/test cases/common/62 string arithmetic/test.json
@@ -0,0 +1,7 @@
+{
+  "stdout": [
+    {
+      "line": "test cases/common/62 string arithmetic/meson.build:25: WARNING: Project targeting '>=0.62.0' but tried to use feature introduced in '0.63.0': multiline format strings."
+    }
+  ]
+}