Moved function definition

[MichalLauer]

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 :)