Add Follow system mode option to Light / Dark theme palette toggle options

[Coderambling]

Adds a Follow system mode option to Light / Dark theme palette toggle options.

Additional icon added for the system preferences option from the same source as the light / dark theme icons:

https://mui.com/material-ui/material-icons/

Assumption

icon library is already referenced in build files, so additional icon should load.

Code changes

Code changes follow Zensical documentation guidelines for adding this mode:

https://zensical.org/docs/setup/colors/?h=.toggle#automatic-light-dark-mode

Section in Zensical docs

# Palette toggle for automatic mode
[[project.theme.palette]]
media = "(prefers-color-scheme)"
toggle.icon = "lucide/sun-moon"
toggle.name = "Switch to light mode"

# Palette toggle for light mode
[[project.theme.palette]]
media = "(prefers-color-scheme: light)"
scheme = "default"
toggle.icon = "lucide/sun"
toggle.name = "Switch to dark mode"

# Palette toggle for dark mode
[[project.theme.palette]]
media = "(prefers-color-scheme: dark)"
scheme = "slate"
toggle.icon = "lucide/moon"
toggle.name = "Switch to system preference"

Remark on syntax currently used in Lumen zensical.toml

The current Lumen zensical.toml file uses a table and sub-table hierarchy:

[[project.theme.palette]]
[project.theme.palette.toggle]

lumen/zensical.toml

Lines 117 to 126 in 562936a

	# Color palette
	[[project.theme.palette]]
	media = "(prefers-color-scheme: light)"
	scheme = "default"
	primary = "custom"
	accent = "blue"
	[project.theme.palette.toggle]
	icon = "material/brightness-7"
	name = "Switch to dark mode"

I have followed that approach in file in this PR.

The Zensical doc combines the table and sub-table in one table. Should this be adopted for the Lumen file?

# Palette toggle for light mode
[[project.theme.palette]]
media = "(prefers-color-scheme: light)"
scheme = "default"
toggle.icon = "lucide/sun"
toggle.name = "Switch to dark mode"

This would apply to the theme.palette section (3 occurences) and the theme.social sections in the Lumen file (5 occurrences).

Lumen extra.social code section

lumen/zensical.toml

Lines 189 to 210 in 562936a

	# Extra configuration
	[project.extra]
	[[project.extra.social]]
	icon = "fontawesome/brands/github"
	link = "https://github.com/holoviz/lumen"
	[[project.extra.social]]
	icon = "fontawesome/brands/discourse"
	link = "https://discourse.holoviz.org/c/lumen/"
	[[project.extra.social]]
	icon = "fontawesome/brands/discord"
	link = "https://discord.com/invite/rb6gPXbdAr"
	[[project.extra.social]]
	icon = "fontawesome/brands/twitter"
	link = "https://twitter.com/holoviz_org"
	[[project.extra.social]]
	icon = "fontawesome/brands/python"
	link = "https://pypi.org/project/lumen/"

Please advise, happy to make a separate PR for this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.34%. Comparing base (562936a) to head (5006542).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1970   +/-   ##
=======================================
  Coverage   72.34%   72.34%           
=======================================
  Files         203      203           
  Lines       35680    35680           
=======================================
+ Hits        25812    25814    +2     
+ Misses       9868     9866    -2     

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[ghostiee-11]

@Coderambling, Can you please use this for Drafting the Pr and also prefer adding short video or images of before and after, https://github.com/holoviz-dev/holoviz-skills/tree/main/contributing-to-holoviz/skills/pr-description