Add begin0-let-to-define-begin0 rule (#263)

In [this code](https://github.com/jackfirth/yaragg/blob/9d48ab7152003e39aa6c1bab11a601d3723143fa/parser-tools/yacc-to-scheme.rkt#L115-L129), there's a `let` expression immediately within a `begin0` expression to compute a variable needed for the result of the `begin0` expression. But if that `let` is lifted up into a `define`, the code becomes far more readable.

default-recommendations/let-binding-suggestions-test.rkt

@@ -503,6 +503,20 @@ test: "variable definition with nested let binding of name bound earlier not ref
 ------------------------------
 
 
+test: "let binding nested in begin0 extractable to definition"
+------------------------------
+(define (f)
+  (begin0 (let ([x 1]) x)
+    (displayln "foo")))
+------------------------------
+------------------------------
+(define (f)
+  (define x 1)
+  (begin0 x
+    (displayln "foo")))
+------------------------------
+
+
 test: "redundant let bindings can be removed"
 ------------------------------
 (define x 1)

default-recommendations/let-binding-suggestions.rkt

@@ -78,6 +78,20 @@
     body-after ...)])
 
 
+(define-definition-context-refactoring-rule begin0-let-to-define-begin0
+  #:description "This `let` expression can be pulled up into a `define` expression."
+  #:literals (begin0 let)
+  [(~seq body-before ...
+         (begin0
+           (~and original-let (let ([nested-id:id nested-expr:expr]) result-expr:expr))
+           body-after ...))
+   #:when (not
+           (set-member? (syntax-bound-identifiers #'(body-before ... body-after ...)) #'nested-id))
+   (body-before ...
+    (define nested-id nested-expr)
+    (begin0 (~replacement result-expr #:original original-let) body-after ...))])
+
+
 (define-refactoring-rule delete-redundant-let
   #:description "This `let` binding does nothing and can be removed."
   #:literals (let)
@@ -90,7 +104,8 @@
   (refactoring-suite
    #:name (name let-binding-suggestions)
    #:rules
-   (list define-let-to-double-define
+   (list begin0-let-to-define-begin0
+         define-let-to-double-define
          delete-redundant-let
          let-to-define
          let-values-then-call-to-call-with-values