fix(options): extend dry-run validation

Problem: nvim_set_option_value() dry runs skip checks in did_set_*
callbacks. This logic also prepares values needed to apply the option,
such as tab widths or a compiled regexp. Moving the checks into
validation callbacks needs a way to save that work for did_set_*.

Solution: Store prepared values in optset_T so did_set_* can consume
them after the option is stored. Dry runs perform the same validation,
then free the prepared values without changing live state.

This extends dry-run validation for:

- 'cedit': reject invalid command-line window keys.
- 'completeitemalign': require each completion column exactly once.
- 'cursorlineopt': reject empty values and conflicting line modes.
- 'encoding': accept only UTF-8 and its aliases.
- 'fileencoding': reject lists and local changes with 'nomodifiable'.
- 'messagesopt': require mandatory fields and enforce numeric limits.
- 'spellcapcheck': return regexp errors, including under :silent!.
- 'spellsuggest': reject conflicting suggestion modes.
- 'varsofttabstop' and 'vartabstop': reject invalid tab widths.
- 'wildmode': reject more than four completion stages.

'makeencoding' also uses the shared encoding preparation.

Separating value preparation exposed a bug in 'vartabstop' and
'varsofttabstop': failed assignments could discard local tab widths, and
global assignments could overwrite them. Fix this by preparing new
widths separately and applying them only after a valid local assignment.

AI-assisted

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