fix: preserve jsdoc markdown hard breaks

[jmolz]

Refs denoland/deno#31831.

JSDoc is commonly rendered as Markdown, but the TypeScript formatter currently trims two trailing spaces from non-final JSDoc lines. That removes Markdown hard line breaks. The Markdown plugin normalizes the same input to a backslash hard break, so this applies the same normalization to JSDoc lines while keeping non-JSDoc block comments on the existing trimming behavior.

The regression coverage also guards the main edge cases from the narrower behavior:

• converts a JSDoc Markdown hard break when another content line follows
• does not convert a hard break on the last meaningful JSDoc content line
• does not convert trailing spaces inside a fenced JSDoc code block
• does not convert hard breaks in non-JSDoc block comments

Tests:

• cargo test --test specs issue031831
• cargo test --test specs JSDocComments_All
• cargo test --test specs CommentBlocks_All
• cargo test