FR: support project-scoped snippets (`.code-snippets`)

The VSCode format also supports [project-scoped snippets](https://code.visualstudio.com/docs/editor/userdefinedsnippets#_project-snippet-scope), which is also supported by [Luasnip](https://github.com/L3MON4D3/LuaSnip/blob/master/DOC.md#standalone).

The syntax is pretty much the same, only with the addition of the `scope` key to determine the filetype of a snippet.

Start with the linked VSCode project-snippet documentation and LuaSnip standalone documentation, then locate the existing VSCode snippet handling in the repository. Done means project-scoped `.code-snippets` files are accepted and their `scope` key determines the snippet filetype.

Written by the indexing model from the issue text.