Problem:
Besides being redundant with vim.iter():flatten(), `tbl_flatten` has
these problems:
- Has `tbl_` prefix but only accepts lists.
- Discards some results! Compare the following:
- iter.flatten():
```
vim.iter({1, { { a = 2 } }, { 3 } }):flatten():totable()
```
- tbl_flatten:
```
vim.tbl_flatten({1, { { a = 2 } }, { 3 } })
```
Solution:
Deprecate tbl_flatten.
Note:
iter:flatten() currently fails ("flatten() requires a list-like table")
on this code from gen_lsp.lua:
local anonym = vim.iter({ -- remove nil
anonymous_num > 1 and '' or nil,
'---@class ' .. anonymous_classname,
}):flatten():totable()
Should we enhance :flatten() to work for arrays?
9912a4c81b