feat: Index behaviors by return typing strategy

* Adds the concept of, and options for, the return typing strategy
  * adds the strategy at the beginning of every behavior index
  * adds the strategy as an additional argument to resolve
  * No actual use of return type strategy so far
  * Sets up eslint
  * Fixes eslint errors

src/core/config.js

@@ -0,0 +1,11 @@
+import {ImplementationsGenerator} from './Implementations.js'
+import {ReturnTyping} from './Type.js'
+import {match, Passthru} from './TypePatterns.js'
+
+export const config = new ImplementationsGenerator(() => match(Passthru, {
+    // default comparison tolerances:
+    relTol: 1e-12,
+    absTol: 1e-15,
+    // Strategy for choosing operation return types:
+    returnTyping: ReturnTyping.free,
+}))