feat(themes): add doom-synthwave84

[jbcorwin]

Adding a direct port of robb0wen's Synthwave '84 theme.

I pulled the exact hex codes from the original VS Code theme to match the aesthetic as much as possible.

Notes:

• Solaire-mode works out of the box (distinct bg and bg-alt colors are mapped so inactive windows dim correctly).
• Terminal fallbacks were added so the background doesn't break in non-GUI setups.
• Standard Doom user toggles (brighter comments, brighter modeline, and padded modeline) were added as well.

Screenshots

Syntax Highlighting

Solaire-Mode (Treemacs vs Active Window)

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to Doom's git conventions.
• I am blindly checking these off.
• This PR contains AI-generated work.
• Any relevant issues or PRs have been linked to.
• This a draft PR; I need more time to finish it.