Go: add cursor helpers for function-body and init-wrapped-if checks

[bmuschko]

What

Adds two reusable cursor predicates to rewrite-go pkg/visitor:

• IsFunctionBodyBlock(c *Cursor) bool — reports whether the block at the cursor is a function's body rather than a nested block such as a loop or if body.
• IsInitWrappedIf(c *Cursor) bool — reports whether the If at the cursor is the inner statement of a golang.StatementWithInit (an if init; cond form).

Why

This is the follow-up from moderneinc/recipes-go#53 (comment), where these helpers were flagged as generic enough to belong upstream rather than duplicated in a downstream recipe repo. Hosting them in the visitor package lets recipes across languages share them. The package already imports both java and golang, so no new import cycle is introduced.

Not upstreamed

• BaseIndent — not needed. rewrite-go already exposes the identical logic as Space.Indent() in pkg/tree/java/space.go, so callers should use that instead of a duplicate.
• IsErrNotNil — stays local to the recipe repo, as it is recipe-specific rather than generic LST navigation.

[greg-at-moderne]

Not sure what is meant by:

Hosting them in the visitor package lets recipes across languages share them.

in the PR description. But feel free to ignore this comment.

[greg-at-moderne]

I am wondering whether we should have more "end-to-end" tests - i.e. a test which would involve parsing a piece of code and assert on count of matches or similar.
As is, the test is rather trivial as it follows the implementation internals.

[bmuschko]

Makes sense. Changed.

[bmuschko]

Hosting them in the visitor package lets recipes across languages share them.

What I was trying to say is that by putting these helpers in the shared pkg/visitor package instead of inside a specific recipe package, any recipe can reuse them rather than each copying its own version (which is what happened in recipes-go).