Add save-name command setting

[rogierx]

Fixes #20.

This adds a tsperf.tracer.saveNameCommand setting so users can choose the initial persistent save by running a shell command in the workspace folder, for example git branch --show-current. The first non-empty stdout line becomes the save name; empty output or command failures fall back to default.

The generated save name is sanitized before it becomes part of the storage path, and the setting is machine-scoped because it executes a local shell command.

Validation:

• pnpm exec vitest run test/index.test.ts
• pnpm run typecheck
• pnpm exec eslint src/saveNameCommand.ts src/appState.ts src/configuration.ts src/constants.ts scripts/generate-contributes.ts test/index.test.ts
• pnpm exec rollup -c
• git diff --check