Problem:
LSP spec uses the term "position encoding" where we say "offset encoding".

Solution:
- Rename it everywhere except `vim.lsp.Client.offset_encoding` (which would be breaking).
- Mention "position encoding" in the documentation for `vim.lsp.Client.offset_encoding`.

An implication of this current approach is that `NVIM_API_LEVEL` should be
bumped when a new Lua function is added.

TODO(future): add a lint check which requires `@since` on all new functions.

ref #25416

Some composite/compound types even as basic as `(string|number)[]` are
not currently supported by the luacats LPEG grammar used by gen_vimdoc.
It would be parsed & rendered as just `string|number`.

Changeset adds better support for these types.

Problem:
While LuaCATS's generics system are still considered WIP by luals, they
currently support type captured generics.

See "Capture with Backtick" example:
https://luals.github.io/wiki/annotations/#generic

Solution:
Add support for it in the LuaCATS grammar

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.

- Tags are now created with `[tag]()`
- References are now created with `[tag]`
- Code spans are no longer wrapped