Problem:
After 25439dad7e, `opts` is optional, but not params that follow it.
Solution:
- Update `gen_api_dispatch.lua`, docs, tests.
- Enhance the `nvim_get_api_info()` api-metadata to include a boolean
flag indicating whether a parameter is optional:
```
[type, name, optional]
```
Note: Currently, optional params are assumed to be Dict/Array. Since we
don't yet have any use-cases of optional params following `opts`, the
build just checks this assumption and we can deal with it later if we
ever care to:
nvim_get_hl: optional param "foo" has type "Integer" but (currently)
we assume Dict/Array
dc8934482e