Add tracking for various unsupported features and implement xsl:output as the one failing the most tests

[bojidar-bg]

After the relatively low counts of fixed tests by #123, I decided to approach the problem a bit more systematically, and find which parts of the code throw the most Unsupported errors throughout. This was done in the first two commits. (First commit should have been part of #118 probably, but apparently I missed updating the filters back then.)

At that point, I could use the following Bash pipeline to determine the most-common sources of errors in the test suite:

cargo run --release --bin xee-testrunner -- -v all vendor/xslt-tests/ | cut -sd' ' -f3- | cut -d'(' -f-2 | sort | uniq -c | sort -h

Output at that point:

<snip>
    923 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Failed parsing XSLT: Unexpected { span: Span { start: 40, end: 51 } }
    984 PASS
   2027 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Failed parsing XSLT: Unsupported("Unsupported declaration: Output")
   5113 CONTEXT ITEM ERROR No stylesheet found

As most errors seemed to come from the lack of xsl:output support (and most tests that use that just have a basic xsl:output method="xml" encoding="UTF-8" /> or so at the start), I decided to follow up on that and implement xsl:output.

Now, the output from the pipeline above looks like the following:

<snip>
    345 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Failed parsing XSLT: Unsupported("Unsupported declaration: Variable")
    449 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Named templates not supported
    466 FAIL xml:
    663 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Failed parsing XSLT: Unexpected { span: Span { start: 40, end: 51 } }
   1098 PASS
   2160 CONTEXT ITEM ERROR Error parsing stylesheet: Unsupported Failed parsing XSLT: Unsupported("Unsupported declaration: Param")
   5113 CONTEXT ITEM ERROR No stylesheet found

So, with this implemented, working through swathes of tests in #123 makes more sense (: