Auto merge of #29486 - petrochenkov:multiwild, r=Manishearth

Motivation:
- It is not actually a pattern
- It is not actually needed, except for...

Drawback:
- Slice patterns like `[a, _.., b]` are pretty-printed as `[a, .., b]`. Great loss :(

plugin-[breaking-change], as always

src/test/run-make/graphviz-flowgraph/f07.dot-expected.dot

@@ -10,7 +10,7 @@ digraph block {
     N8[label="(dummy_node)"];
     N9[label="local x"];
     N10[label="local y"];
-    N11[label="pat .."];
+    N11[label="pat _"];
     N12[label="pat [x, y, ..]"];
     N13[label="expr x"];
     N14[label="expr y"];