feat: `--[no]-trailing-slash` CLI flags

[andybarron]

Changes

• Add --trailing-slash and --no-trailing-slash CLI flags, which correlate to "trailingSlash": true and "trailingSlash": false in serve.json.
• Update affected tests.
• Add tests for new CLI flags.

Rationale

I ran into #628 while using serve to view Jest coverage reports. I was able to bypass the issue using serve.json, but I needed to put it outside the coverage/ directory (which is in .gitignore) and use the -c flag to link it. Felt like overkill for a pretty simple flag, so I added it to the CLI!