Allow disabling single & block quotes completely

[eemeli]

This PR contains a breaking change for how singleQuote: false is handled. To retain the previous behaviour, use singleQuote: null instead or leave the option unset.

Adds a ToString option blockQuote: boolean | 'folded' | 'literal' and expands singleQuote: boolean | null, such that using false for either prevents that scalar style from being used at all in the output.

For blockQuote, setting either the 'folded' or 'literal' style will use it for all scalar values for which block quotes are used.

For singleQuote, the default null value will prefer double-quoted but allow single-quoted where aesthetically more pleasing. Setting this to true will prefer single-quoted instead.

It is intentional that no option exists for disabling plain or double-quoted scalar styles, as they're required for properly representing all scalar values.