Add progressive JPEG file to test suite and test that loading it returns the expected `Unsupported_Frame_Type` error.
This JPEG variant will hopefully be supported in the future, but we should at least return an error rather than use `unsupported()`.
`c/frontend/tokenizer`:
add proper "Example:" header to demo example code,
removed empty lines.
`container/bit_array`:
moved comment before package;
aligned narrative lines to left margin;
converted case lines into bulleted lines ("- ");
converted individual examples to single-tab-indented preformatted text.
`dynlib`:
removed "//+build ignore" line;
added newline at EOF.
`image/netpmb`:
converted indented lines of "Reading", "Wrting" and "Some syntax..." into bulleted lists;
"Formats" indented lines kept as they are as the preformatted text seems relevant to keep the alignments;
doubly indented lines kept as single-indented to keep them different (as the format does not allow for two-level bulleted lists);
removed empy lines.
`os/os2`: WIP, not modified
`sys/info`:
removed "//+build ignore" line;
converted tab-indented initial description into regular left-margin comment;
moved uncommented sample code within the doc comment as an "Example:";
moved simple- and double-tabbed separate comments with sample Windows and macOS outputs within the doc comment as bulleted headlines with preformatted output listings;
removed now empty comments and blank lines after the package line.
`text/i18n`:
removed "//+build ignore" line;
moved the pacakge line at the end;
de-indented the tab-indented introductory narrative;
moved sample code comments into the doc comment as tab-indented code with a proper "Example:" heading;
removed "```" MD attempts at code formatting.
`text/table`:
unindented the comment lines of a descriptive kind;
headlines of major subdivisions are marked as bold;
kept code samples as tab-indented preformatted text (as there are several of them, the standard "Example:" and "Output:" headings cannot be used) removing the "```" MD attempts at code formatting;
removed in-between blank lines.
