www/novnc: Improve port

• Use DISTVERSIONPREFIX
• Utilize EXTRACT_AFTER_ARGS to reduce I/O
• Isolate COPYTREE_SHARE line in do-install
• Remove commands related to .travis.yml .eslintignore .eslintrc, these files are no longer packaged

References:
https://docs.freebsd.org/en/books/porters-handbook/book/#makefile-master_sites-github
https://docs.freebsd.org/en/books/porters-handbook/book/#install-copytree

PR: 280603
Approved by: portmgr (maintainer timeout, 2+ weeks)