Add `infix fun String.selfieIs` (similar to Kotest `shouldBe`)

[nedtwigg]

Kotest has a very nice shouldBe method:

someFunction() shouldBe "some value"

It would be handy if Selfie had an infix version:

someFunction() selfieIs null // equivalent to `toBe_TODO`, run and it becomes
someFunction() selfieIs "some value"

And all the //selfieonce, //SELFIEWRITE would work too if implemented roughly like so:

// in com.diffplug.selfie
infix fun String.selfieIs(expected: String?) : Unit {
  if (expected == null) {
    Selfie.expectSelfie(this).toBe_TODO()
  } else {
    Selfie.expectSelfie(this).toBe(expected)
  }
}

// in com.diffplug.selfie.coroutines
suspend infix fun String.selfieIs(expected: String?) : Unit {
  if (expected == null) {
    Selfie.expectSelfie(this).toBe_TODO()
  } else {
    Selfie.expectSelfie(this).toBe(expected)
  }
}

The main challenge will be adapting this code to parse infix multiline literals

selfie/jvm/selfie-lib/src/commonMain/kotlin/com/diffplug/selfie/guts/SourceFile.kt

Lines 105 to 111 in 952a4eb

	fun parseToBe_TODO(lineOneIndexed: Int): ToBeLiteral {
	return parseToBeLike(".toBe_TODO(", lineOneIndexed)
	}
	fun parseToBe(lineOneIndexed: Int): ToBeLiteral {
	return parseToBeLike(".toBe(", lineOneIndexed)
	}
	private fun parseToBeLike(prefix: String, lineOneIndexed: Int): ToBeLiteral {