[wasm-split] Allow splitting globals

[biggs0125]

The wasm-split tool only has support for splitting functions but it does not support splitting globals. Rather than leave every global in the main module as an export, ideally the tool could detect which globals are only used in specific submodules and split those out as well.

[tlively]

cc @aheejin.

This is also true of e.g. tables, memories, tags, etc.