feat: add terminal_features config for user-configurable terminal feature support

[Copilot]

• src/terminal/writer.go — invert SupportsFeature: Features now maps program → features
• src/terminal/iterm.go — remove ITermFeatures type and gob.Register; rewrite RenderItermFeatures using SupportsFeature; add HasITermFeatures() guard
• src/config/config.go — remove ITermFeatures field; update Fish PromptMark check to scan TerminalFeatures
• src/prompt/primary.go — replace ITermFeatures != nil && isIterm() guard with HasITermFeatures()
• src/terminal/writer_test.go — update TestSupportsFeature cases for inverted map shape; add iterm feature cases
• themes/schema.json — remove iterm_features; update terminal_features description
• website/docs/configuration/general.mdx — remove iterm_features row; update terminal_features row
• Build and tests pass; CodeQL clean

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[heaths]

@JanDeDobbeleer so you're open to the terminal_features idea? I also wasn't sure whether it should be feature then terminal, or terminal and features. Either way, given the pace some terminals are moving (iTerm2 is always improving, and Ghostty is a fast up-and-comer - or like Windows Terminal did for quite a while not long ago) this is problem better than trying to keep OMP up to date.