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, 42 insertions, 1 deletions

diff --git a/test cases/common/62 string arithmetic/meson.build b/test cases/common/62 string arithmetic/meson.build
index 8ef350ede..194df0d9a 100644
--- a/test cases/common/62 string arithmetic/meson.build
+++ b/test cases/common/62 string arithmetic/meson.build
@@ -1,4 +1,4 @@
-project('string arithmetic', 'c')
+project('string arithmetic', 'c', meson_version: '>=0.62.0')
 
 assert('foo' + 'bar' == 'foobar')
 assert('foo' + 'bar' + 'baz' == 'foobarbaz')
@@ -6,3 +6,44 @@ assert('foo' + 'bar' + 'baz' == 'foobarbaz')
 a = 'a'
 b = 'b'
 assert(a + b + 'c' == 'abc')
+
+# ------------------------------------------------------------------------------
+# format strings:
+# ------------------------------------------------------------------------------
+sub1 = 'the'
+sub2 = ' quick\n'
+sub3 = '    brown'
+sub4 = '\nfox'
+x = f'@sub1@@sub2@@sub3@@sub4@'
+
+assert(x == sub1 + sub2 + sub3 + sub4)
+assert(x == 'the quick\n    brown\nfox')
+
+# ------------------------------------------------------------------------------
+#  multi-line format strings
+# ------------------------------------------------------------------------------
+y_actual = f'''This is a multi-line comment with string substition:
+  "@sub1@@sub2@@sub3@@sub4@"
+
+And I can even substitute the entry multiple times!
+
+@sub1@
+@sub2@
+@sub3@
+'''
+
+y_expect = '''This is a multi-line comment with string substition:
+  "the quick
+    brown
+fox"
+
+And I can even substitute the entry multiple times!
+
+the
+ quick
+
+    brown
+'''
+message('actual=' + y_actual)
+message('expect=' + y_expect)
+assert(y_actual == y_expect)