Add system dependant version of String.trimIndent()

[alexander-yevsyukov]

We have String.ti() extension function which handles the issue with hardcoded line separators in Kotlin string utilities.

We also need analogue for the String.trimIndent() function, which does the same. This would be specifically useful for tests related to code generation.

The implementation is trivial:

public fun String.ti(): String = trimMargin().fixLineEndings()

We also probably may want to open access to fixLineEndings() which is now private to the file.

[alexander-yevsyukov]

Resolved by #810.