feat(snippet): snippets may be marked as hidden.

They won't show up in completion results of engines respecting the key.

DOC.md

@@ -56,6 +56,8 @@ entries:
                `dscr`. Overrides docstrings loaded from json.
 - `docTrig`: string, for snippets triggered using a lua pattern: define the
              trigger that is used during docstring-generation.
+- `hidden`: hint for completion-engines, if set, the snippet should not show
+            up when querying snippets.
 
 `s` can also be a single string, in which case it is used instead of `trig`, all
 other values being defaulted:

lua/luasnip/nodes/snippet.lua

@@ -132,6 +132,7 @@ local function S(context, nodes, opts)
 		dependents = {},
 		active = false,
 		type = types.snippet,
+		hidden = context.hidden,
 	})
 	-- is propagated to all subsnippets, used to quickly find the outer snippet
 	snip.snippet = snip