Reduce unused exports

@rebornix and I were wondering if there is any dead code in the VS Code codebase, and one idea was to detect unused exports across the codebase, since unused variables are flagged by the compiler but unused exports are not. I ran `ts-unused-exports` on our sources and generated the following list. I've already done a pass to remove some false positives but apologies if several remain. NB: The primary purpose of this issue is to identify unused functions and classes which aren't being picked up by the compiler because they're exported and which can be safely deleted from our sources to reduce our bundle size. A secondary benefit is to remove unused types and interfaces, e.g. anything which was exported at one point and then ceased to be referenced from other files (which more impacts compile time for dev workflows and pipeline runs, and maybe TS server analysis time, but has no direct impact on bundle size since types are erased). If you export a type or interface that is not currently actively imported, but is used in the public API of a class that you expose, then it's quite likely that you want to keep that type/interface exported and need not un-export it as part of this issue. Please feel free to remove your assignment if you find that this isn't valuable. | Git blame | Function | File | | -- | -- | -- | | @sandy081 | `IButtonWithDropdownOptions` |[src/vs/base/browser/ui/button/button.ts](https://insiders.vscode.dev/github/microsoft/vscode/blob/main/src/vs/base/browser/ui/button/button.ts)| | @sandy081 | `PartMatches` |[src/vs/base/browser/ui/keybindingLabel/keybindingLabel.ts](https://insiders.vscode.dev/github/microsoft/vscode/blob/main/src/vs/base/browser/ui/keybindingLabel/keybindingLabel.ts)| | @sandy081 | `Matches` |[src/vs/base/browser/ui/keybindingLabel/keybindingLabel.ts](https://insiders.vscode.dev/github/microsoft/vscode/blob/main/src/vs/base/browser/ui/keybindingLabel/keybindingLabel.ts)| | @sandy081 | `defaultStyles` |[src/vs/base/browser/ui/selectB…