Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: famiu <famiuhaque@protonmail.com>

Problem:
Using `meths.exec2("code", { output = true })` is too verbose.

Solution:
Use exec_capture() in more places.

Problem:
The signature of nvim_exec() is not extensible per ":help api-contract".

Solution:
Introduce nvim_exec2() and deprecate nvim_exec().

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

Problem:    #{g:x} was seen as a curly-braces expression.
Solution:   Do never see #{} as a curly-braces expression. (closes vim/vim#11075)
https://github.com/vim/vim/commit/7c7e1e9b98d4e5dbe7358c795a635c6f1f36f418

Problem:    Cannot use expand() to get the script name.
Solution:   Support expand('<script>'). (closes vim/vim#10121)
https://github.com/vim/vim/commit/6013d0045dec7ca7c0068fbe186c42d754a7368b

Use `.sn_name` instead of `->sn_name` as v8.2.0154 hasn't been ported.
Cherry-pick builtin.txt expand() doc from latest Vim.