Problem:
`vim.diagnostic.set()` doesn't actually accept a list of
`vim.Diagnostic` as internally `vim.diagnostic.set()` normalizes the
diagnostics and this normalization is assumed throughout the module.
Solution:
- Add a new type `vim.Diagnostic.Set` which is the input to `vim.diagnostic.set()`
- `col` is now an optional field and defaults to `0` to be consistent
with `vim.diagnostic.match()`.
- Change `table.insert(t, x)` to `table[#table + 1] = x` for improved
type checking.
533cc0ab35