Fix test generation placeholders inside and adjacent to RegExp literals

[ptomato]

Several improvements to the test generation tool. The first commit is based on @guijemont's work and fixes the case where a placeholder comment is immediately adjacent to the end of a RegExp literal.

The second commit adds a feature to test template files which can be used as an escape hatch in case the /* and */ delimiters of placeholder comments would be misparsed. (This is one way that it could work, I'm not attached to it.)

Closes: #3808

[ptomato]

@jugglinmike I see your point, in fact I shared your skepticism about the configurable delimiters feature when Richard suggested it. This comment explains why I changed my mind. But like I said in the description, I'm not attached to the design of this feature, so I'd be happy to go with something completely different.

But more immediately: the first commit actually doesn't unblock #3807 unless I add some more processing for comments inside regexp literals. (The first commit fixes /foo//*{ bar }*/ but breaks /foo/*{ bar }*/baz/. Fixing the latter made me come around to Richard's suggestion of configurable delimiters.) I'd be happy to make a separate PR with 83614a1 and a second commit that fixes comments inside regexp literals, while we iterate on the configurable delimiters, or decide not to do them.