chore: add funding.json (#5581)

## Description

This adds a FUNDING.json file to the main repo. This is required for
various projects, however the immediate case is for registering for an
optimism retropgf grant.

---------

Co-authored-by: João Oliveira <[email protected]>

Author: AgeManning

FUNDING.json

@@ -0,0 +1,5 @@
+{
+  "opRetro": {
+    "projectId": "0xdf1bb03d08808e2d789f5eac8462bdc560f1bb5b0877f0cf8c66ab53a0bc2f5c"
+  }
+}