Simplify setup code

[creativenull]

Omit init() and setup() just like it was done in creativenull/efmls-configs-nvim#30.

The reason being that having two separate functions to have the user setup is not very friendly, the preference would be to keep setup code as minimal as possible, at the same time, provide more flexibility to the user if in case they might want to adjust some settings.

But since the API is different in diagnostic-languageserver compared to efm-langserver, I'm thinking of creating a wrapper to generate the LSP config and then have the user merge them on lspconfig.diagnosticls.setup().

New proposal example:

local eslint = require('diagnosticls-configs.linters.eslint')
local prettier = require('diagnosticls-configs.formatters.prettier')

local diagnosticls_config = require('diagnosticls-configs').create({
  javascript = {
    linters = { eslint },
    formatters = { prettier },
  },
  typescript = {
    linters = { eslint },
    formatters = { prettier },
  },
})

require('lspconfig').diagnosticls.setup(vim.tbl_extend('force', diagnosticls_config, {
  on_attach = on_attach,
  capabilities = capabilities,
})

For defaults:

local diagnosticls_config = require('diagnosticls-configs').defaults()

To add additional languages or to overrides defaults, just pass the same table structure as in create():

local diagnosticls_config = require('diagnosticls-configs').defaults({
  javascript = {
    linters = { eslint },
    formatters = { prettier },
  },
})