Add suport for handling unbound prefixes on tag names and attribute keys

[duanyang25]

Describe the PR

• This PR adds support for handling unbound prefixes on Issue Unbound prefixes not handled #1341 about tag names and attribute keys as mentioned.
• I have not changed any original functionality, but I have introduced new getter functions Tag.unicodeName() and Attribute.getConvertedKey() to get the converted tag names and attribute keys with namespaces whose unrecognized symbols, like : or other symbols inside the tag names, are converted in Unicode 16 starting from U000000 to U10FFFF following the standard.
• Currently, I have handled the first case mentioned on Issue Unbound prefixes not handled #1341, the test:h1 to testU00003Ah1. And I am still working on the second case regarding xlink. Would someone give me some suggestions to handle the second case? It will be appreciated.
• I have handled both cases following HTML Living Standard https://html.spec.whatwg.org as SimonSchmid mentioned and suggested in the issue ticket.

Hi there, I am an undergraduate student. One of my courses this semester related to Software Engineering requires us to fix issues on Github, and hopefully, these pull requests could be merged into the project.

I have worked on this issue for more than two weeks. Please review this pull request and feel free to edit these codes I submitted. Thank you very much!

Related issues

• Fix Unbound prefixes not handled #1341

Ready?

• I have added unit tests for this issue Unbound prefixes not handled #1341.
• I have passed all unit tests including both existing ones and ones newly written by me by running mvn install.
• I have successfully built Jsoup.
• I have added (in-code) documentation following the same style of other functions in the same file.