feat: replace CodeMirror in side editor with Shiki-based textarea

[KermanX]

A breaking change because old custom styles may no longer work.

Pros

• Reduce bundle size
• Highlighting is more accurate
• No CJS problems introduced by CodeMirror
• Load faster
• Easier to maintain

Cons

• Undo/redo functionality becomes weak
• I have to use word-break: break-all, otherwise the text inside <pre shiki> and <textarea> will be mismatched, because <span>s in <pre shiki> affects the word breaking.

Why not use monaco-editor

• If monaco-editor is used in the side editor, it has to be loaded in dev, even the user doesn't use {monaco-*} features
• Kind of too complex. Most of the features aren't useful in the side editor.

Resolve #1391.

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