Add start_in_insert option

[mbenford]

Add a new option, start_in_insert, that defaults to true and controls whether the input will start in insert or normal mode.

Context

I'd like to conditionally enable normal mode when certain criteria are met, e.g. when using vim.ui.input with LSP rename.

Description

This PR introduces an option that controls whether insert mode will be activated or not, and it can be used along with get_config for conditional behavior. For instance:

require('dressing').setup({
  input = {
    ...
    get_config = function(opts)
      return {
        start_in_insert = #(opts.default or '') == 0,
      }
    end,
  }
})

Test Plan

Test case 1

• Set start_in_insert to true in setup() (or leave it unset since the default value is true already).
• Run vim.ui.input({default = "some text"}, function() end).
• An input window should open in insert mode.

Test case 2

• Set start_in_insert to false in setup()
• Run vim.ui.input({default = "some text"}, function() end).
• An input window should open in normal mode.

[stevearc]

Looks good, thanks for the PR!