Switch from XML to xml2

[burgerga]

Currently htmltab is removed from CRAN (#1) due to an error popping up in one of the unit tests:

Version: 0.8.2
Check: tests
Result: ERROR
     Running ‘spelling.R’
     Running ‘testthat.R’ [4s/18s]
    Running the tests in ‘tests/testthat.R’ failed.
    Complete output:
     > library(testthat)
     > test_check("htmltab")
     Loading required package: htmltab
     [ FAIL 1 | WARN 15 | SKIP 1 | PASS 112 ]
    
     ══ Skipped tests ═══════════════════════════════════════════════════════════════
     • On CRAN (1)
    
     ══ Failed tests ════════════════════════════════════════════════════════════════
     ── Error ('test_expand_spans.R:257'): http://stackoverflow.com/questions/24215584/html-complex-tables ──
     Error in `if (e$code == xmlParserErrors["XML_IO_LOAD_ERROR"]) stop(e)`: argument is of length zero
     Backtrace:
     ▆
     1. ├─XML::htmlParse(stack) at test_expand_spans.R:257:2
     2. └─XML (local) `<fn>`(...)

(full report at https://cran-archive.r-project.org/web/checks/2022/2022-11-11_check_results_htmltab.html)

This is a regression caused by some combination of R > 4.2.2 and XML, and is not due to htmltab, however, XML is in keep-alive mode so I don't expect changes in XML, so the suggestion by CRAN (de facto XML maintainer) was to move from XML to xml2.

This will be quite a lot of work...
Some tips to get started here: https://gist.github.com/nuest/3ed3b0057713eb4f4d75d11bb62f2d66