Factor out parseYAMLBoolean from core.getBooleanInput

[andreasabel]

The (relatively new) function getBooleanInput in core bundles the parsing of a YAML boolean with getInput. However, that parsing functionality would be useful on its own.
I suggest to factor out function parseYAMLBoolean(text: string): boolean from getBooleanInput such that the latter is essentially just a composition of parseYAMLBoolean and getInput:

function getBooleanInput(name: string, options?: InputOptions): boolean { 
  try {
    return parseYAMLBoolean(getInput(name, options));
  } catch ... {
   // Throw the informative error referring to "name" here
  }
}

toolkit/packages/core/src/core.ts

Lines 164 to 181 in b2d865f

	* Gets the input value of the boolean type in the YAML 1.2 "core schema" specification.
	* Support boolean input list: `true | True | TRUE | false | False | FALSE` .
	* The return value is also in boolean type.
	* ref: https://yaml.org/spec/1.2/spec.html#id2804923
	*
	* @param name name of the input to get
	* @param options optional. See InputOptions.
	* @returns boolean
	*/
	export function getBooleanInput(name: string, options?: InputOptions): boolean {
	const trueValue = ['true', 'True', 'TRUE']
	const falseValue = ['false', 'False', 'FALSE']
	const val = getInput(name, options)
	if (trueValue.includes(val)) return true
	if (falseValue.includes(val)) return false
	throw new TypeError(
	`Input does not meet YAML 1.2 "Core Schema" specification: ${name}\n` +
	`Support boolean input list: \`true | True | TRUE | false | False | FALSE\``