Treesitter parser (#2902)

A push towards better editor support.
See utils/treesitter/README.md

---------

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

explorer/BUILD

@@ -80,3 +80,17 @@ filegroup(
     # Files are used for validating fuzzer completeness.
     visibility = ["//explorer/fuzzing:__pkg__"],
 )
+
+filegroup(
+    name = "treesitter_testdata",
+    srcs = glob(
+        ["testdata/**/*.carbon"],
+        exclude = [
+            "testdata/**/fail_*",
+
+            # multiline strings
+            "testdata/string/*",
+        ],
+    ),
+    visibility = ["//utils/treesitter:__pkg__"],
+)