fix(markup): parse \(...\) inline math next to letters and digits (#39773)

Fixes https://github.com/go-gitea/gitea/issues/39770

`\(x^2\)is` and `\(x^2\)123` rendered as plain text because `\(...\)`
went through the same check on surrounding characters as `$...$`. That
check is there to skip things like `US$1`, which can't happen with
explicit `\(` `\)` delimiters, so it's now skipped for them, the same as
for "$`...`$". `$x$a` still isn't parsed.

modules/markup/markdown/math/inline_parser.go

@@ -91,6 +91,7 @@ func (parser *inlineParser) Parse(parent ast.Node, block text.Reader, pc parser.
 	} else {
 		startMarkLen = 2
 		stopMark = parser.endBytesParentheses
+		checkSurrounding = false // explicit delimiters like "$`...`$", adjacent text is unambiguous
 	}
 
 	if line[0] == '$' && !parser.enableInlineDollar && (len(line) == 1 || line[1] != '`') {