[formatter] Avoid spurious line breaks after inline HTML elements

Previously, HTMLFormatter.#afterCloseTag unconditionally inserted a
newline after every closing tag, even for leaf elements without child
elements. When a leaf element was followed by text on the same line
(such as `<span>Some text</span>}` or `<li> hello <b>world</b>!`), the
trailing text was forced onto a new line.

Update #afterCloseTag to skip adding a newline when a leaf element is
followed by inline text (either directly or after horizontal
whitespace on the same line).

R=kimanh@chromium.org

Fixed: 388055639
TAG=agy
CONV=f1f9f717-124c-4523-ba24-a08b1b90dea7
Change-Id: I68f1b27cad9787b0239ca0c1a09d42b83b52d992
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8549245
Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Commit-Queue: Kim-Anh Tran <kimanh@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>