Implement merging of partnered schemas

[happy5214]

Partnered schemas were introduced in HED standard schema version 8.2.0. Currently, hed-javascript has basic support for using partnered schemas separately from each other, with different prefixes for each schema. Now, we require support for so-called "lazy partnered schema" merging, where compatible partnered schemas with the same prefix may be merged into a single schema.

Partnered schemas are compatible (in this version) if:

• They have the same withStandard XML attribute (referencing the same standard schema version; merging this standard schema version shall be a no-op).
• Their list of tags with the inLibrary schema attribute have no overlap.

A sketch for merging partnered schemas A and B (assuming they are both partnered to a standard schema we're calling S:

1. Parse A and B as normal. Failures here end the process.
2. Verify that A and B are both actually partnered to the same standard schema (S in this case). Error out if they differ.
3. For the following steps, assume we're merging B into A (tags only for now). Iterate through B's tags.
   1. If the tag does not have an inLibrary schema attribute, skip the tag. (It is already in the standard schema S.)
   2. If B's short tag name is already in A, error out (error message: "Library A and Library B are incompatible and a prefix must be given for one or the other."). This includes short tags that resolve to the same full path.
   3. Otherwise, add the tag from B to A at the position indicated by the rooted schema attribute (if the attribute is not present, place it at the top level).