TypeError: ts2.isNamedTupleMember is not a function

dts throws TypeError when typescript is not installed

the error message could be more friendly

```
npx tsup src/index.ts --format cjs,esm --dts

CLI Building entry: src/index.ts
CLI Using tsconfig: tsconfig.json
CLI tsup v6.5.0
CLI Target: esnext
CJS Build start
ESM Build start
ESM dist/index.js 84.52 KB
ESM ⚡️ Build success in 637ms
CJS dist/index.cjs 85.36 KB
CJS ⚡️ Build success in 640ms
DTS Build start
TypeError: ts2.isNamedTupleMember is not a function
at DeclarationScope.convertTypeNode (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:6738:17)
at DeclarationScope.convertMembers (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:6692:18)
at Transformer.convertClassOrInterfaceDeclaration (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:7001:15)
at Transformer.convertStatement (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:6942:23)
at new Transformer (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:6899:16)
at convert (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:6890:27)
at Object.transform (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:7165:29)
at transformFile (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:7255:46)
at Object.writeFile (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/rollup.js:7278:27)
at Object.writeFile (node_modules/.pnpm/typescript@3.9.10/node_modules/typescript/lib/typescript.js:16631:14)
Error: error occured in dts build
at Worker. (node_modules/.pnpm/tsup@6.5.0/node_modules/tsup/dist/index.js:2226:26)
at Worker.emit (node:events:513:28)
at MessagePort. (node:internal/worker:238:53)
at [nodejs.internal.kHybridDispatch] (node:internal/event_target:734:20)
at exports.emitMessage (node:internal/per_context/messageport:23:28)
DTS Build error

```