## What is this PR for?
The symbols-outline extra was removed in favor of outline.nvim in #2535
(thanks!), but the configuration for symbols in outline.nvim [is not
backwards-compatible](https://github.com/hedyhli/outline.nvim/issues/12).
This fixes the configuration for the symbols icons and filter to be
usable by outline.nvim.
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## Does this PR fix an existing issue?
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It doesn't seem like anyone has encountered this issue, but I can
confirm that the config currently used by LazyVim is incorrect. The
symbols table is at `symbols` for symbols-outline.nvim, but it's now at
`symbols.icons` for outline.nvim.
There is no such `symbols_blacklist` key. Instead, `symbols.filter` is
used, which is a kind of "whitelist".
Coincidentally, outline.nvim fully supports the LazyVim `kind_filter`
config table structure. It can either be a list of strings (kinds), or a
list of strings for each filetype key. Setting to nil or false makes it
so all symbols are included, just like in LazyVim.
See [the docs on the
`symbols.filter`](https://github.com/hedyhli/outline.nvim?tab=readme-ov-file#symbols-table)
structure.
## Checklist
- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.
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