feat(copmletion): Try to add `Emmet LSP server` completion support

[yebt]

Hi. I will try to make a solution for #16. I apologize in advance for the inconvenience caused and if the code does not follow best practices.

With this patch i try to add the new behavior for the completion suggestions provided by emmet-lsp-server and the generated snippets for this server.

I tried to add the following behavior:

• when the item is preselected caused by vim.opt.completeopt not contain the option noselect, the first item is selected, show the info
• Add peview for the snippet

Before

After

• Use the label for the word item to replace when change the items

Before

After

• Expand items from emmet-lsp

Before html:5 item generate:

a item generate:

ul>li*3 item generate:

After

html:5 item generate:

a item generate:

ul>li*3 item generate:

[glepnir]

quick look is good