Require opam-version 2.1 and greater at the start of the file

[dra27]

opam 2.1 does not define any new fields, so there is no need for any opam file to contain opam-version: "2.1". Given this general quietness, there is one semantic change being made for the 2.1 and subsequent formats in order to allow the lexer and parser to evolve in future. opam-version must appear first (apart from whitespace and comments) for "2.1" and later, or the file does not parse. This gives the possibility in the future that additional constructs can be added to the file format, while retaining compatibility with older lexers.

This PR enforces two requirements on the whole-file lexing:

1. opam-version: "2.1" must be the first non-whitespace/comment field
2. For opam-version: "2.1" and greater, repeating the opam-version field is a parsing error

In both these cases, for maximum API compatibility, Parsing.Parse_error is raised rather than a new exception. API changes in the future will be needed to take advantage of this, but the key thing in this PR is to enforce the invariant so that this can be done when it's needed in future.

Opam itself only uses OpamParser.FullPos.string, OpamParser.FullPos.channel, OpamParser.FullPos.main all of which obey this. The only additional function used is OpamParser.FullPos.value_from_string - when lexing changes are introduced, functions such as that will need a parameter to specify the permitted lexing version.

On top of this, there is then an additional future-proofing change to the library: if opam-version is greater than the library version, then the lexer and parser do a best-effort parse. In this case, Parse_error is only raised if opam-version is not first (since it must be 2.2+) or if it is repeated before the first lexer/parser error. This allows opam (and any other tool) to display a superior "this is an opam 2.2 file, so was ignored" rather than a lexer/parser error message which is more likely to result in users reporting bugs (to pinned repositories, opam-repository maintainers, etc.).

I added a few tests to ensure the Parse_error exceptions are raised. It could do with some more tests, although I have test_ed_ it! There is an additional slightly weird test which ensures that OpamBaseParser.version matches the build version.

[AltGr]

LGTM. Feels a bit weird to have the parser explicitely refer to a specific field by string, with "opam" in its name;
nevertheless I think this was the right thing to do!