Promote decorationIfAbsent to API

[kashike]

Promote into StyleSetter.

adventure/api/src/main/java/net/kyori/adventure/text/format/StyleImpl.java

Lines 339 to 369 in 22f8eca

	// todo(kashike): promote to public api?
	@NotNull Builder decorationIfAbsent(final @NotNull TextDecoration decoration, final TextDecoration.@NotNull State state) {
	requireNonNull(state, "state");
	if (decoration == TextDecoration.BOLD) {
	if (this.bold == TextDecoration.State.NOT_SET) {
	this.bold = state;
	}
	return this;
	} else if (decoration == TextDecoration.ITALIC) {
	if (this.italic == TextDecoration.State.NOT_SET) {
	this.italic = state;
	}
	return this;
	} else if (decoration == TextDecoration.UNDERLINED) {
	if (this.underlined == TextDecoration.State.NOT_SET) {
	this.underlined = state;
	}
	return this;
	} else if (decoration == TextDecoration.STRIKETHROUGH) {
	if (this.strikethrough == TextDecoration.State.NOT_SET) {
	this.strikethrough = state;
	}
	return this;
	} else if (decoration == TextDecoration.OBFUSCATED) {
	if (this.obfuscated == TextDecoration.State.NOT_SET) {
	this.obfuscated = state;
	}
	return this;
	}
	throw new IllegalArgumentException(String.format("unknown decoration '%s'", decoration));
	}