fix(tailwindcss): revert broken config detection #4376

Problem:
`find_tailwind_global_css` attempted to address #4204, where `experimental.configFile` was set using the return value of `vim.fs.find()`.
The language server rejected this with `Invalid experimental.configFile
configuration, not initializing` because `configFile` expects either a string or
a key-value record (object), not an array/list. This was a syntax issue, not
a detection issue.

Using the correct syntax for `configFile` in Lua should be
sufficient to address the original issue. Right now, `find_tailwind_global_css`
always runs for users who haven't explicitly set `configFile` — overriding the
LSP's native detection and **forcing anyone who wants to opt out to manually set
all entry-points by hand.**

Solution:
- Remove `find_tailwind_global_css` entirely and restores `configFile` to its
  default `nil` so the `tailwindcss` LSP handles project detection natively.
- Simplify `before_init` based on [this suggestion from the initial
  PR](https://github.com/neovim/nvim-lspconfig/pull/4222#discussion_r3018499628).

The following syntax worked for me while testing to explicitly set the
`configFile` based on the [official
docs](https://github.com/tailwindlabs/tailwindcss-intellisense#tailwindcssexperimentalconfigfile)
for single entry-point:

> [!NOTE]
> Single entry-point is resolved relative to the workspace root (`root_dir` — verify with `:checkhealth vim.lsp`)

```lua
vim.lsp.config('tailwindcss', {
  settings = {
    tailwindCSS = {
      experimental = {
        -- v3: config file
        configFile = 'tailwind.config.js',
        -- v4: CSS entry-point
        -- configFile = 'src/styles/app.css',
      },
    },
  },
})
```

For projects with multiple entry-points, or different projects, the following
syntax can be used for multiple entry-points:

> [!NOTE]
> Keys are relative to `root_dir` as above, but from my testing on macOS, absolute paths worked better

```lua
vim.lsp.config('tailwindcss', {
  settings = {
    tailwindCSS = {
      experimental = {
        configFile = {
          ['tailwind.config.js'] = '/Users/username/path/to/project-a/**',
          ['src/main.css'] = '/Users/username/path/to/project-b/**',
        },
      },
    },
  },
})
```

#### Project or Local Configuration

For project-specific settings without modifying your global Neovim config:

1. Enable in your Neovim config:
   ```lua
   vim.o.exrc = true
   ```
2. Create `.nvim.lua` in the project root:
   ```lua
   vim.lsp.config('tailwindcss', {
     settings = {
       tailwindCSS = {
         experimental = {
           configFile = 'tailwind.config.ts',
         },
       },
     },
   })
   ```
3. Open `.nvim.lua` and run `:trust` to allow the file, then restart Neovim.
4. Verify with `:checkhealth vim.lsp`.

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