scripts: add version mapping automation v2

Adds lib/version_mapping.rb

Built around treating the file as an array of strings, finding the version mapping table by known header, and then replacing the content after updating & sorting in memory.

Process:

• load version_mappings.md into VersionMapping::Document
• locate and parse the table from document into VersionMapping::Table
• append to the table, where a matching chart Version will overwrite an existing entry by the same version
• write back to version_mappings.md, automatically sorting table

vm = VersionMapping.new(filepath) # will instantiate Document
vm.insert_version(chart_version,app_version) # insert / update versions
vm.finalize #perform write

Missing:

• RSpec (#1193)

Closes #999 (closed)