feat(lsp): add support of LSP items post-processing and deduplication

[sergluka]

Description

LSP references result deduplication. When multiple LSP servers are used, their functionality may overlap, leading to duplicated values in the summarized list. This fix enables either automatic result deduplication or a post-processing function, allowing users to implement custom logic for the resulting list.

A new configuration value, post_process, has been added.

Note: There is a similar MR (#3353), but this approach provides more control to users.

Fixes: #3328

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

• :Telescope lsp_references
• :Telescope lsp_definitions
• :Telescope lsp_type_definitions
• :Telescope lsp_implementations

All lists shouldn't have duplicates

Configuration:

• Neovim version (nvim --version): v0.10.2
• Operating system and version: Ubuntu 22.04.5 LTS

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)