feat(hooks): add useSystemTheme hook for system color scheme detection

[somyabhalani]

Summary

Adds a new useSystemTheme hook that detects the user's system color scheme
preference and automatically updates when the preference changes.

Usage

import { useSystemTheme } from "@blueprintjs/core";

const theme = useSystemTheme(); // "dark" | "light"

Checklist

• Includes tests
• Update documentation

[palantirtech]

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Description

feat(hooks): add useSystemTheme hook for system color scheme detection

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[somyabhalani]

Generate changelog in packages/core/changelog/@unreleased

Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description feat(hooks): add useSystemTheme hook for system color scheme detection

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