feat: restore the module docstring code snippet (#7030)

We had this in mathlib3, allowing one to type `mo` (then press tab, at least for me) in an empty file and have a snippet template of a module docstring inserted with the title named to the filename, which can save a little time.
The implementation is now a bit overkill as it splits on `_` and capitalizes still to get from the filename to the module title, which likely has no affect for most lean 4 filenames.

.vscode/module-docstring.code-snippets

@@ -0,0 +1,35 @@
+{
+	"Module docstring for mathlib": {
+		"scope": "lean4",
+		"prefix": "module docstring",
+		"body": [
+			"/-!",
+      "# ${TM_FILENAME_BASE/([^_]*)(_?)/${1:/capitalize}${2:+ }/g}",
+			"",
+			"## Main definitions",
+			"",
+			"* `FooBar`",
+			"",
+			"## Main statements",
+			"",
+			"* `fooBar_unique`",
+			"",
+			"## Notation",
+			"",
+			"",
+			"",
+			"## Implementation details",
+			"",
+			"",
+			"",
+			"## References",
+			"",
+			"* [F. Bar, *Quuxes*][bibkey]",
+			"",
+			"## Tags",
+			"",
+			"Foobars, barfoos",
+			"-/",
+			"",
+		]},
+}