feat: add Dameng DM8 URL support - #53

Summary

• Register dm as the canonical Dameng DM8 scheme with dm8 and dameng aliases.
• Generate native DM8 DSNs with TCP, localhost, port 5236, credentials, schema, and native query options.
• Reject malformed, repeated, case-colliding, or otherwise unrepresentable URL input instead of silently changing connection semantics.
• Document the Dameng scheme and Go driver in the supported-driver table.
• Keep dburl independent from usql and the Dameng Go driver.

This is the prerequisite URL contract for xo/usql#585.

Verification

• go test ./... -count=1 (221 passed)
• go test -race ./... -count=1 (221 passed)
• CGO_ENABLED=0 go test ./... -count=1
• go vet ./...
• golangci-lint v2.1.6 run ./... (0 issues)
• no new module dependencies; tests use only the Go standard library