Fix plusType for schema that contains only ISL 2.0 version marker

[popematt]

Issue #, if available:

Fixes #305
Also related to #304

Description of changes:

There was incorrect logic for handling appending a new type to a schema in SchemaImpl_2_0.plusType(). The idx == isl.lastIndex meant that the new type would be inserted before the last value, no matter what it was. So, when the existing schema had only one top-level value (in this case, the $ion_schema_2_0 version marker), the new type would be inserted before the version marker, which is illegal.

Now, the logic for adding the type in plusType() is as follows:

1. If we find a type with the same name, replace it with the new type
2. If we find a schema footer, insert the new type immediately before the schema footer
3. Otherwise, append to the very end of the existing schema.

(This is what it was intended to be all along.)

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:
None

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bbab678) 83.18% compared to head (d6aa6d1) 83.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   83.18%   83.24%   +0.05%     
==========================================
  Files         160      160              
  Lines        3777     3777              
  Branches      907      907              
==========================================
+ Hits         3142     3144       +2     
  Misses        360      360              
+ Partials      275      273       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.