feat: expose pendingReload to consumers

[schalk-b]

Description

Implements #4100

Expose pendingReload to consumers

At the moment, when I run a Jest test for the first time, when unoptimized dependencies are discovered the server gets flagged to do a "reload", where it optimize the dependencies and re-bundles the cache.

This results in tests failing, because they are unable to resolve dependencies in the cache while things are reloading.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

This looks like an interesting feature. We'll discuss it next week with the team.

[schalk-b]

Thank you, I'm also not completely convinced that this is the best solution to the problem. There might be better way to address #4100

[haoqunjiang]

I think we need to note here that the pendingReload field might be refreshed. That is, if the user visits a new route, and new dependencies are discovered, it may be re-assigned with a new Promise.

[schalk-b]

I don't think this is the right solution to the problem. This doesn't handle with concurrency / locking of the .vite directory.