Add mermaid-scale option to resize Mermaid images - #15192

tests: tests/smoke/render/render-mermaid-scale.test.ts now imports the standard library directly (testing/asserts); if tests/asserts.ts exports what this file uses with the same behaviour, import it from there. Otherwise nothing to change in this PR: a maintainer accepts the new count before merging (see the job summary).

tests: +1 Deno.readTextFileSync in tests/smoke/render/render-mermaid-scale.test.ts; src/deno_ral has no replacement yet, so nothing to change in this PR. A maintainer accepts the new count before merging (see the job summary).