Cache `getExePath()` for performance reasons

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Would it be possible to have `getExePath()` cache its result so we don't have to do all this lookup every time `tsgo` is invoked? The platform and arch rarely changes once installed on disk.

In one of the repositories I'm maintaining, we've inlined what `tsgo.js` does and added caching on top of it. We're not a very big monorepo, but this shaved off several seconds for us.

从 tsgo.js 开始，定位 getExePath() 并跟踪调用 tsgo 时它运行的次数。检查现有查找行为在不同平台和架构上的表现，然后确认缓存能够保留所选的可执行文件，同时避免重复查找。完成的标准是：重复调用不再重新执行查找，且现有行为保持不变。

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