- Move all generator Lua scripts to the `src/gen/`
- Add a `.luarc.json` to `src/gen/`
- Add a `preload.lua` to `src/gen/`
  - Add `src` to `package.path` so it aligns with `.luarc.json'
- Fix all `require` statements in `src/gen/` so they are consistent:
    - `require('scripts.foo')` -> `require('gen.foo')`
    - `require('src.nvim.options')` -> `require('nvim.options')`
    - `require('api.dispatch_deprecated')` -> `require('nvim.api.dispatch_deprecated')`

Problem:
`termopen` has long been a superficial wrapper around `jobstart`, and
has no real purpose. Also, `vim.system` and `nvim_open_term` presumably
will replace all features of `jobstart` and `termopen`, so centralizing
the logic will help with that.

Solution:
- Introduce `eval/deprecated.c`, where all deprecated eval funcs will live.
- Introduce "term" flag of `jobstart`.
- Deprecate `termopen`.

In Vim a lot of filesystem functions have been moved to filepath.c.
However, some of these functions actually deal with file contents, and
Nvim's filesystem-related functions are spread out in a different way.
Therefore, it's better to use a different file for these functions.

Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.

Solution:
- Enable lintlua for `src/` directory.

followup to 517f0cc634b985057da5b95cf4ad659ee456a77e

Problem:    The mouse code is spread out.
Solution:   Move all the mouse code to mouse.c. (Yegappan Lakshmanan,
            closes vim/vim#4959)

https://github.com/vim/vim/commit/b20b9e14ddd8db111e886ad0494e15b955159426

Also move getmousepos() there.

N/A patches for version.c:

vim-patch:8.1.2070: mouse code is spread out

Problem:    Mouse code is spread out.
Solution:   Move mouse terminal code parsing to mouse.c. (Yegappan Lakshmanan,
            closes vim/vim#4966)

https://github.com/vim/vim/commit/b8ff5c271ee5dcef6f63436b77e228e062ff9a0e

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Cannot check the current state.
Solution:   Add the state() function.

https://github.com/vim/vim/commit/0e57dd859ecb1e8a3b91509d2f4343e839340eb8

Co-authored-by: Bram Moolenaar <Bram@vim.org>

- eval.lua is now the source of truth.

- Formatting is much more consistent.

- Fixed Lua type generation for polymorphic functions (get(), etc).

- Removed "Overview" section from builtin.txt
  - Can generate this if we really want it.

- Moved functions from sign.txt and testing.txt into builtin.txt.

- Removed the *timer* *timers* tags since libuv timers via vim.uv should be preferred.

- Removed the temp-file-name tag from tempname()

- Moved lueval() from lua.txt to builtin.txt.

* Fix indent

* fixup!

* fixup! fixup!

* fixup! better tag formatting

* fixup: revert changes no longer needed

* fixup! CI

---------

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

Problem:    The evalfunc.c file is too big.
Solution:   Move undo functions to undo.c.  Move cmdline functions to
            ex_getln.c.  Move some container functions to list.c.

https://github.com/vim/vim/commit/08c308aeb5e7dfa18fa61f261b0bff79517a4883

Undo functions only. Cmdline functions have already been moved.
A lot of container functions have been added to eval/funcs.c instead of
list.c in previously ported Vim 8.2.x patches, so deal with that later.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Using inclusive index for slice is not always desired.
Solution:   Add the slice() method, which has an exclusive index. (closes
            vim/vim#7408)

https://github.com/vim/vim/commit/6601b62943a19d4f8818c3638440663d67a17b6a

Cherry-pick a line in docs added later.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Functions for string manipulation are spread out.
Solution:   Move string related functions to a new source file. (Yegappan
            Lakshmanan, closes vim/vim#8470)

https://github.com/vim/vim/commit/a2438132a675be4dde3acbdf03ba1fdb2f09427c

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>