Add WithPadding for independent top and bottom banner padding

[polymorcodeus]

Summary

Adds WithPadding(top, bottom int) so callers can set top and bottom banner padding independently. Keeps WithZeroPadding() as a compatibility wrapper for WithPadding(0, 0). Closes#8.

What's new

• Independent padding control - WithPadding(top, bottom int) sets the number of leading and trailing newlines separately.
• Backward-compatible helper - WithZeroPadding() now delegates to WithPadding(0, 0) and continues to disable the default top padding.
• Bottom padding support - Banner.BottomPadding holds the trailing newline count and is applied in CmdBanner.

Behavior changes / guardrails

• Banner.TopPadding changed from bool to int to support multiple leading newlines; existing callers using the functional options are unaffected.
• NewCmdBanner defaults to TopPadding: 1 and BottomPadding: 0, preserving the previous single-leading-newline default.
• Negative padding values loop zero times (Go's for i := 0; i < n semantics), so callers should pass non-negative integers.

Testing

• Added banner_test.go with tests for default padding, WithPadding, top padding, bottom padding, and zero padding.
• go test ./..., go vet ./..., and go fmt ./... are all green.