Consider split expr.proto into multiple files

[advancedxy]

What is the problem the feature request solves?

I just noticed that the generated ExprOuterClass.java is too large(~2.61MB), which makes IntelliJ IDEA stops indexing on that file.

I think we should consider split the expr.proto into multiple files and reduces the generated code file length?

Describe the potential solution

1. split the expr.proto into multiple files
2. a temporary workaround, set IDEA's advanced property: idea.max.intellisense.filesize to a larger than, such as 9999 may work around this issue. For how to set this properties, please refer https://www.jetbrains.com/help/idea/tuning-the-ide.html#c61775a4_106

cc @sunchao @viirya

Additional context

No response