Form Block: Decrease delay before child block block inserter icon appears

### Impacted plugin

Jetpack

### What

When moving the mouse between two child blocks in the Form block, there is a delay in showing the block inserter:

https://user-images.githubusercontent.com/16754605/198331804-68afc21c-88c9-4d14-b378-08aedd0c1a6b.mov

Decreasing the delay may make it easier to notice the block inserter when moving the mouse regularly.

Suggested in internal testing: p8oabR-Ym-p2#comment-6726

The issue concerns the Jetpack Form block's child-block inserter while moving between child blocks. Start by locating the Form block implementation and the delay controlling the inserter icon; verify completion by confirming the icon appears sooner during normal pointer movement.

Written by the indexing model from the issue text.