feat: Add mode to inject templates after marker

[anthonychung14]

Closes #127

Adds a mode param that lets user inject code after the marker.

This has proven to be useful if we want to use a more understandable marker such as

at: "bob := map[string]Foo{"

rather than something more brittle/less understandable than

at: "&SomeRandomItemInList{}"

Also, in order to have tests pass on my machine, I had to change go.mod per instructions here golang/go#65568 (comment)

I can remove this, but I'm not super proficient at Go, so if there's another workaround, happy to remove this commit

[anthonychung14]

@hay-kot - wdyt? would like to use this. think it works given the test case.

[hay-kot]

This looks good! I want to refactor some of those tests (not your code, my code) before I merge so give me a couple days and we'll get this in!