Change FormattedString's end field to a Literal

I noticed this while working on the documentation. I think this predated
pyre's Literal support.

I only did this for `end` and not `start`, because there's a large
combinatorial set of valid `start` values. If we wanted to better type
`start`, we should probably first separate the prefix from the opening
quotes.

libcst/_nodes/_expression.py

@@ -763,7 +763,7 @@ class FormattedString(_BasePrefixedString):
     start: str = 'f"'
 
     #: The trailing quote. This must match the type of quote used in ``start``.
-    end: str = '"'
+    end: Literal['"', "'", '"""', "'''"] = '"'
 
     lpar: Sequence[LeftParen] = ()
     #: Sequence of parenthesis for precidence dictation.

libcst/_nodes/tests/test_atom.py

@@ -839,12 +839,6 @@ class AtomTest(CSTNodeTest):
                 ),
                 "expected_re": "must have matching enclosing quotes",
             },
-            {
-                "get_node": (
-                    lambda: cst.FormattedString(start="f'''", parts=(), end="''")
-                ),
-                "expected_re": "must have matching enclosing quotes",
-            },
             {
                 "get_node": (
                     lambda: cst.ConcatenatedString(