Scaladoc tool: fix parsing bug which could cause very slow performance or incorrect output

[liang3zy22]

Fix scala/bug#12509

I think I found the root cause. I used a simple test case to debug this issue:

/**
* {{{foo}}}}
*/

Not sure the fix and/or commit message is completely correct.
So please give comment.

[lrytz]

LGTM, thank you. Test case:

/**
 * {{{
 * code
 * before }}} after
 */
class A

The output is broken before your patch.

[Jasper-M]

Same issue in Scala 3 https://github.com/lampepfl/dotty/blob/3.2.1-RC2/scaladoc/src/dotty/tools/scaladoc/tasty/comments/Preparser.scala#L61

[liang3zy22]

@Jasper-M, can I create a PR for scala3(lampepfl/dotty) to fix this issue? I haven't done it before.

[Jasper-M]

@liang3zy22 Sure, it's at least as easy as it is for Scala 2.

[SethTisue]

Yes, that would be appreciated!

[som-snytt]

it's at least as easy as it is for Scala 2

It's easier because issues aren't split into a different repo.