[0074] 在 (scheme base) 中导出 call/cc，撰写对应的单元测试（包含文档）

[da-liii]

变更内容

• 完善 tests/scheme/base/call-slash-cc-test.scm，补充 call/cc 的文档和丰富测试用例
• 新增 tests/scheme/base/call-with-current-continuation-test.scm，说明与 call/cc 为同一过程，并提示参考 gf doc "call/cc"
• 新增 devel/0074.md 任务文档

验证

bin/gf test tests/scheme/base/call-slash-cc-test.scm
bin/gf test tests/scheme/base/call-with-current-continuation-test.scm
bin/gf doc "call/cc"
bin/gf doc "call-with-current-continuation"