Express validity, error correction and percent encode sets in a single table

[alwinb]

This is a proposal to include a table, either as clarification,
or (my current preference) even as a full replacement for describing:

• Percent encode sets
• Valid vs invalid individual code points per component, and
• Error correction behaviour of the above,

Within a single small-ish table.

For each component of an URL that contains a percent encoded string,
we can describe per codepoint its validity, error correction and encoding.

A single code point is either:

• v: Valid and included verbatim in the output URL.
• E: (Escape) valid but nonetheless percent encoded.
• T: (Tolerate) invalid, but nonetheless left untouched by the parser —resulting in an invalid URL as output.
• F: (Fixed) invalid and fixed by the parser (and setters) by percent encoding the occurrence.
• R: (Reject) Invalid and causing a hard error, so that they do not end up in output URLs.

Notes:

• 'Other control' here is control-c0 ∪ del-c1 ∪ surrogate ∪ non-char
• The apostrophe in the query is special cased for 'non-special' URLs where it is left untouched (ie. v: Valid) hence the superscript. Special query could also be broken out into a separate column.

(If there have been changes to these sets in the last year or so, the table might be slightly out of date)

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I would like to thank @LEW21 for the idea to chart things out in this way in #379