Skip string literal docblock when an unresolved value is passed, add --literals option

[TomasVotruba]

Two changes around the @param 'a'|'b' string literal docblock from #38.

Fix: unresolved arguments now block the docblock

Arguments collect could not resolve (e.g. a variable) were dropped, so "only literals were passed" really meant "only literals among the resolvable calls". On mautic, Download::setTrackingId() got @param '123456'|'13'|... from test values, while AssetModel passes a variable.

Collect now records such arguments with an empty type. Aggregate ignores them for the native type, but they drop the literal docblock.

New --literals option

argtyper add-types . --literals

Only adds string types that come with the literal docblock, nothing else.

On mautic: 92 literal docblocks before the fix, 39 after.