feat(diagnostic): prefer the signs from vim.diagnostic.config()

[DOD-101]

Using vim.fn.sign_define for diagnostic signs is deprecated.

Description

This commit will still fall back on those deprecated sign definitions,
if the new ones created via vim.diagnostic.config() are not defined.

This code is largely taken from #3146
with minor adjustments to actually make it work this time.

Please include a summary of the change and which issue is fixed. Please also
include relevant motivation and context

Related to this issue: #3464 (they ended up using vim.fn.sign_define)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list relevant details about your configuration

1. Setup vim.diagnostic

vim.diagnostic.config({
	signs = { text = { "󰅚", "󰀪", "󰌶", "" } }
})

2. Setup an LSP to get diagnostic messages
3. Open diagnostic picker

Configuration:

• Neovim version (nvim --version):

NVIM v0.11.3
Build type: Release
LuaJIT 2.1.1741730670

• Operating system and version:

NixOS 25.11 (Xantusia) x86_64
Linux 6.12.41

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)