api: Obsolete spacing and stop passing it to liblouis

[henrikottesorensen]

Fixes main, which has been red since #31 merged.

liblouis 3.39.0 deprecated the spacing parameter of lou_translate, lou_translateString, lou_backTranslate and lou_backTranslateString, and in practice it now ignores it. Each function only logs "the spacing parameter is deprecated and ignored; pass NULL" and never writes to the buffer. The release notes only say deprecated; the source shows it's ignored.

That breaks #19 on 3.39.0:

• OutputSpacing reads back from a buffer liblouis no longer writes, so it returned the caller's own request as if it were liblouis's answer.
• Every call that passed spacing logged the warning.
• Spacing_ReportsTheRequestedDigits_AgainstTheOutputCellsTheyProduced and Spacing_BackTranslation_CoversTheWholeOutput fail on main.

Changes

• C# can't put [Obsolete] on a parameter, so each of the four public Translate/BackTranslate methods gets a new overload without spacing. The new overload holds the body.
• The existing overloads that take spacing are marked [Obsolete] and call the new ones without it. Callers keep compiling, get a CS0618 warning, and get the same results.
• liblouis is always passed NULL for spacing, so nothing gets logged. The P/Invoke declarations are unchanged apart from their docs.
• The obsolete overloads no longer throw when spacing's length doesn't match the input, since the value is never used.
• Removed PrepareSpacingBuffer, ExtractOutputSpacing and TranslatedString.OutputSpacing from Marshal spacing as the output buffer it is #19. OutputSpacing never shipped in a release (no tags or packages exist) and would always be null now.
• SpacingTests now covers the new behaviour:
  • Each obsolete overload returns the same output and positions as its replacement.
  • A spacing of the wrong length is accepted.
  • liblouis logs no spacing warning, checked against a log message that is known to arrive.
• All other tests use the new overloads.

Testing

• 80/80 passing on net8.0 and net10.0 against the 3.39.0 natives and tables, with no compiler warnings.
• I checked that the no-warning test can fail: passing a spacing buffer to lou_translateString makes it fail.

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