refactor: move build errors out of schema

[matt2e]

This is in preparation for a full language.proto spec for language plugins.
Changes:

• Adds language.proto with only error specs
• Adds builderrors package to separate build errors out of schema
• Use builderrors.Error values directly, rather than pointers
• builderrors.Position has StartColumn and EndColumn, rather than builderrors.Error having schema.Pos and EndColumn.
  • Convert to an error position by using schema.Pos.ToErrorPos() or schema.Pos.ToErrorPosWithEnd(column)
• Error.Pos.String() skips the end column if its the same as the start column. 25:7-7 -> 25:7

[stuartwdouglas]

I don't think the skip-proto-breaking label takes effect until there is a change to the PR.