Migrate `PerCharacterEscaper` from JVM-only to multiplatform

[nedtwigg]

Inside the snapshot-lib project you will see these folders:

• commonMain - compiles for all platforms
• jvmMain - compiles only on the JVM
• jsMain - compiles only to JS
• commonTest - these tests run on all platforms
• jvmTest - these tests only run on JVM
• etc.

Good example

We want as much code as possible to live in common. A good example of multiplatform is this:

• a simple internal expect fun to wrap a platform-specific API
• all the logic of ArrayMap lives in common

https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/commonMain/kotlin/com/diffplug/snapshot/ArrayMap.kt#L22-L115

Then we implement the "expect" for both jvm and js with "actual"

• https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/jvmMain/kotlin/com/diffplug/snapshot/FastMap.jvm.kt#L25-L26
• https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/jsMain/kotlin/com/diffplug/snapshot/FastMap.js.kt#L20-L31

Work to be done.

In this case, we have JVM-only code. It should be pretty easy to move it into common, but there are a few tricky parts, which I don't want to deal with right now. So I move only the public API into common.

https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/commonMain/kotlin/com/diffplug/snapshot/PerCharacterEscaper.kt#L18-L21

Leave the implementation as jvm only, and the js just throws TODO exceptions for now

• https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/jvmMain/kotlin/com/diffplug/snapshot/PerCharacterEscaper.jvm.kt#L18-L171
• https://github.com/diffplug/spotless-snapshot/blob/f63192a84390901a3d3543066d095ea23bf81d21/snapshot-lib/src/jsMain/kotlin/com/diffplug/snapshot/PerCharacterEscaper.js.kt#L18-L21

To implement this I would

• move PerCharacterEscaperTest out of jvmTest and into commonTest
  • gradlew jvmTest will still be passing, but gradlew jsTest will now be failing
• remove the expect actual stuff and just put the whole JVM implementation into common
• fix compile errors with as little platform-specific code as possible

[nedtwigg]

@jknack if you're eager to work on something and/or learn more about Kotlin Multiplatform, this issue is ready to complete now or anytime in the future.

Shipping a complete working JVM library is the first step, and that's blocked on me right now. You can always convert our existing stuff to JS/multiplatform as we go, and that will be very useful later on.

[nedtwigg]

Implementation in progress at #9