feature: add support for threads in linux

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

This PR adds process thread support for just Linux (default off, can be enabled with --get-threads). I can explore this for other operating systems later.

Examples:

With tree on:

Tree off:

Issue

If applicable, what issue does this address?

Part of #228

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[jamartin9]

htop decided against this in favour of PF_KTHREAD (freebsd can use sysctl mib afaik for it)

[ClementTsang]

Noted, thanks!

[Copilot]

Pull Request Overview

This PR adds support for collecting and displaying process threads on Linux platforms through a new --get-threads command line option. The feature is disabled by default and only available on Linux.

Key changes include:

• New thread collection and display functionality for Linux processes
• UI styling for thread entries to visually distinguish them from regular processes
• Configuration options and command-line flags for enabling thread support

Reviewed Changes

Copilot reviewed 28 out of 29 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/collection/processes/linux/process.rs	Core process parsing logic updated to collect thread information from /proc/<PID>/task
src/collection/processes/linux/mod.rs	Process collection loop modified to handle threads as separate entries
src/collection/processes.rs	New ProcessType enum to categorize processes vs threads
src/widgets/process_table/process_data.rs	Process widget data structure updated with thread styling support
src/options/args.rs	New --get-threads command line argument added
src/options/config/process.rs	Configuration support for thread collection setting
src/canvas/components/data_table/data_type.rs	DataToCell trait extended with cell-level styling capabilities
Multiple theme files	Thread-specific text styling colors added across all themes

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