`yxx` in Normal mode over a Lua or Vimscript code block section will execute the code.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Design goals/requirements:
- Default configuration of a server can be distributed across multiple sources.
  - And via RTP discovery.
- Default configuration can be specified for all servers.
- Configuration _can_ be project specific.

Solution:

- Two new API's:
  - `vim.lsp.config(name, cfg)`:
    - Used to define default configurations for servers of name.
    - Can be used like a table or called as a function.
    - Use `vim.lsp.confg('*', cfg)` to specify default config for all
      servers.
  - `vim.lsp.enable(name)`
    - Used to enable servers of name. Uses configuration defined
    via `vim.lsp.config()`.

* refactor(shared): extract `vim._list_insert` and `vim._list_remove`

* feat(lsp): add `vim.lsp.foldexpr()`

* docs(lsp): add a todo for state management

* feat(lsp): add `vim.lsp.folding_range.foldclose()`

* feat(lsp): schedule `foldclose()` if the buffer is not up-to-date

* feat(lsp): add `vim.lsp.foldtext()`

* feat(lsp): support multiple folding range providers

* refactor(lsp): expose all folding related functions under `vim.lsp.*`

* perf(lsp): add `lsp.MultiHandler` for do `foldupdate()` only once

Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.