Deprecate es6-style functions and fdc3Ready function in methods.ts

[kriswest]

The methods.ts file in the FDC3 codebase provides wrapper functions FDC3 Desktop Agent API calls, allowing individual calls to be imported from the library. These calls check that window.fdc3 is present and resolve to an error if not, via an fdc3Ready function. The purpose of that function is superseded by the new getAgent function specified in the FDC3 for the Web PR #1191, whilst a reference to an individual function from the Desktop Agent API can also be achieved by destructuring.

As discussed at #1400, these calls appear to be of no significant utility (from FDC3 2.2 onwards) and they should be deprecated and removed in a future FDC3 major version (FDC3 3.0).