**Problem:** `vim.treesitter.get_parser` will throw an error if no parser
can be found.
- This means the caller is responsible for wrapping it in a `pcall`,
which is easy to forget
- It also makes it slightly harder to potentially memoize `get_parser`
in the future
- It's a bit unintuitive since many other `get_*` style functions
conventionally return `nil` if no object is found (e.g. `get_node`,
`get_lang`, `query.get`, etc.)
**Solution:** Return `nil` if no parser can be found or created
- This requires a function signature change, and some new assertions in
places where the parser will always (or should always) be found.
- This commit starts by making this change internally, since it is
breaking. Eventually it will be rolled out to the public API.
b9b408a56c