Support random color scheme selection

[RAGHAV-0202]

Summary

Closes #9422

Introduces a special _random keyword for the colorScheme setting. When colorScheme is set to "_random" in settings.json, a randomly chosen color scheme from the available schemes is applied each time a new tab or pane is created.

This is inspired by Oh-My-Zsh's random theme feature, as suggested in the original issue.

Usage

In settings.json, set:

"colorScheme": "_random"

Each new tab will now open with a randomly selected color scheme.

Changes

CascadiaSettings.cpp

• Added RandomSchemeToken constant (L"_random")
• Modified _validateAllSchemesExist() to skip validation when the scheme name is _random, preventing the UnknownColorScheme warning that blocked the previous attempt at this feature

TerminalSettings.cpp

• Modified _ApplyAppearanceSettings() to detect the _random token and resolve it to a randomly selected scheme from the available color schemes map
• Uses std::random_device + std::mt19937 for proper random selection

Design Decisions

• Used _random (with underscore prefix) as the token per @zadjii-msft's suggestion that it should be "something slightly more obscure than just random"
• Random selection happens per-tab (each new tab gets a freshly random scheme), matching the issue's request
• Both dark and light color scheme names support the _random token independently

Testing

• This is my first contribution to the project. I'm working on macOS and relying on CI for build verification.
• Manual test: Set "colorScheme": "_random", open multiple tabs, verify each gets a different random scheme and no warnings appear.

[RAGHAV-0202]

@microsoft-github-policy-service agree

[RAGHAV-0202]

Hi @zadjii-msft, this PR implements the _random color scheme as discussed in #9422 (including skipping validation and resolving a random scheme at runtime).

Would really appreciate your feedback when you get time. Thanks!