Moved function definition

Hello, the only purpose of this PR is to change the order of function definition. Currently, `processWidget()` is used in function `widgetFunc()`, but only after it's definition. This makes the package https://github.com/rstudio/shinytest2 fail because it does not allow global variables. I am aware that this issue is more related to `{shinytest2}` rather than to `{DT}`, but the issue there is stale (https://github.com/rstudio/shinytest2/issues/330). As this change does not affect any functionality, I thought it could be fixed right here. If I should update also the package version/NEWS or something similar, please, feel free to let me know. If you do not like this change, I will understand. Thanks for this package and your hard work :)