Migrate Stringifiers to Kotlin

[alexander-yevsyukov]

This PR:

• Transforms the utility Java class Stringifiers to Kotlin object, adding convenience extension functions to it.
• Adds stringify() method per Simplify Stringifier.toString call in Java #799.
• Updates the dependency on Protobuf to version 3.25.0. See note on this below.

Updated usage of protobuf dependency

Version 3.25.0 of Protobuf added .proto files to com.google.protobuf:protobuf-kotlin artifact. Now .proto files are available from both com.google.protobuf:protobuf-java and the Kotlin artifact.

We used to add all the Protobuf libraries to the protobuf dependency for simplicity. SInce only one library (protobuf-java) had .proto source code, it did not cause any problems, until the 3.25.0 update.

The solution is to pass only protobuf-java to the protobuf dependency.

Other changes

• The Text class was deprecated in favor of the Spine Text library.
• Iterable.joinByLines() Kotlin ext. fun was introduced to be used instead of Joiner on System.lineSeparator() wherever possible.
• Top-level Kotlin functions were added to provide idiomatic syntax when using Stringifiers.

[codecov]

Codecov Report

Merging #806 (85213a4) into master (3c0eb04) will decrease coverage by 0.10%.
The diff coverage is 87.09%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #806      +/-   ##
============================================
- Coverage     74.64%   74.55%   -0.10%     
+ Complexity     1144     1129      -15     
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  Files           180      180              
  Lines          4205     4189      -16     
  Branches        335      335              
============================================
- Hits           3139     3123      -16     
  Misses          938      938              
  Partials        128      128