Replace sources selection with new selection List/Dictionary & FOSS model support

[w0rp]

NOTE: This is a breaking change, and we're early enough into the project to make these breaking.

Solving this issue will help to solve #21 and we should throw in making the URL for the APIs configurable as to support FOSS OpenAI compatible APIs.

We currently configure sources like so in Vim:

let g:neural = {
\   'source': {
\       'openai': {
\           'api_key': $OPENAI_API_KEY,
\       },
\   },
\}

And also in Lua:

require('neural').setup({
  ui = {
    animated_sign_enabled = false,
  },
  source = {
    openai = {
      api_key = vim.env.OPENAI_API_KEY,
    },
  },
})

We then select a source like so:

g:neural.selected                                           *g:neural.selected*
                                                        *vim.g.neural.selected*
Type: |String|
Default: `'openai'`

Selects a source for Neural to use. Possible values are:

1. `'openai'` - OpenAI
2. `'chatgpt'` - ChatGPT

Expressed in Lua syntax alone, we should instead make our sources configurable this way:

require('neural').setup({
  ui = {
    animated_sign_enabled = false,
  },
  source = {
    type = 'openai',
    api_key = vim.env.OPENAI_API_KEY,
    url = 'https://custom-url.com',
  },
})

Setting the source itself should set the selection. We need to add a url setting to the existing sources to make it possible to select FOSS models. We should additionally support this in future, but not now (separate issue).

All possible fields should always support defaults at all times, and the type should determine the default values we select. The values should be printed back to the user so you can see exactly what is selected, including when you :echo g:neural.

require('neural').setup({
  ui = {
    animated_sign_enabled = false,
  },
  source = {
    {
      type = 'openai',
      url = 'https://custom-url-1.com',
    },
    {
      type = 'openai',
      url = 'https://custom-url-2.com',
    },
  },
})

[w0rp]

The plugin could explain back to the user automatically when no source is selected that they need to set one, and point them to the documentation on how to do so.

[Jorenar]

It would be also helpful when using enterprise instances