Support case-insensitive literal tokens

[alllex]

Fixes #9

[alllex]

I realized that it's possible to use receiver properties as default values for extension functions parameters. This way, the switch to enable case-insensitivity can be located in a single place -- object MyGrammar : Grammar<T>(ignoreCase = true) and functions like Grammar<*>.literalToken would just pick it up.

You can see an example of this in a test:

parsus/src/commonTest/kotlin/me/alllex/parsus/Tests.kt

Lines 497 to 517 in 5d77cd4

	fun ignoreCaseGrammar() {
	object : Grammar<SyntaxTree>(ignoreCase = true) {
	val lit by literalToken("a")
	val reLit by regexToken("[bc]")
	val re by regexToken(Regex("[de]"))
	val lam by token { s, i -> if (s[i] == 'f' || (ignoreCase && s[i] == 'F')) 1 else 0 }
	val lamStrict by token { s, i -> if (s[i] == 'g') 1 else 0 }
	override val root by parser { lexeme(lit) } or parser { lexeme(reLit) } or parser { lexeme(re) } or parser { lexeme(lam) } or parser { lexeme(lamStrict) }
	}.run {
	assertParsed("a").isEqualTo(lit.lex("a"))
	assertParsed("A").isEqualTo(lit.lex("A"))
	assertParsed("b").isEqualTo(reLit.lex("b"))
	assertParsed("C").isEqualTo(reLit.lex("C"))
	assertParsed("D").isEqualTo(re.lex("D"))
	assertParsed("e").isEqualTo(re.lex("e"))
	assertParsed("f").isEqualTo(lam.lex("f"))
	assertParsed("F").isEqualTo(lam.lex("F"))
	assertParsed("g").isEqualTo(lamStrict.lex("g"))
	assertThat(parseEntire("G")).failedWith(NoMatchingToken(0))
	}
	}