Add hover provider

[milesziemer]

Issue #, if available:
#41
Description of changes:
This PR adds the hover action to the LSP, with tests for both IDL 1.0 and 2.0.
Notes:

• IDL 1.0 tests expect rendered markdown with IDL 2.0 syntax because the model serializer used only generated IDL 2.0 syntax, and there is no IDL 1.0 model serializer. A model serializer is required, as opposed to just rendering the text from the source file, because the rendered markdown may contain more information than is in the text of the source file, and because shapes pulled in as external dependencies would not have accessible source files.
• Hover support for elision syntax is not included in this PR and is being tracked in Jump to definition and hover support for elided members #62

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