Add keymaps for LSP document highlight

[peter-bread]

document_highlight()                        *vim.lsp.buf.document_highlight()*
    Send request to the server to resolve document highlights for the current
    text document position. This request can be triggered by a key mapping or
    by events such as `CursorHold`, e.g.: >vim
        autocmd CursorHold  <buffer> lua vim.lsp.buf.document_highlight()
        autocmd CursorHoldI <buffer> lua vim.lsp.buf.document_highlight()
        autocmd CursorMoved <buffer> lua vim.lsp.buf.clear_references()
<

    Note: Usage of |vim.lsp.buf.document_highlight()| requires the following
    highlight groups to be defined or you won't be able to see the actual
    highlights. |hl-LspReferenceText| |hl-LspReferenceRead|
    |hl-LspReferenceWrite|

One keymap to highlight, another to clear references.

Perhaps in the first, it should clear then highlight so you don't have multiple items highlighted.

[peter-bread]

local x

x = 9

x = 0

x = x + 12

local function check()
  local ns = vim.api.nvim_create_namespace("nvim.lsp.references")
  local exts = vim.api.nvim_buf_get_extmarks(0, ns, 0, -1, { details = true })

  return exts
end

local exts = check()
local ext = exts[1] or nil

if ext then
  local row = ext[2]
  local col = ext[3]

  vim.api.nvim_win_set_cursor(0, { row + 1, col })
end