Add static type for Cadence external PathLink value

[fxamacker]

Issue To Be Solved

cadence.PathLink doesn't have a static type. It is needed because:

• CCF specs (draft) needs to include cadence.PathLink static type.
• CCF codec (WIP) needs all Cadence external values to have a static type.

Updates #2157

Suggested Solution

Maybe add PathLinkType and return PathLinkType from PathLink.Type().

EDIT: Renamed Link to its new name PathLink.

[turbolent]

I might have commented on this somewhere before, but I don't think that this is actually needed:
After some digging, support for exporting links was implemented very early on and is/was likely only needed for the Playground. I can't think of a way when a link would be exported, as links are stored internally, but are not first-class values that could be imported or exported by a user.

[SupunS]

I initially had the same thought, but would this be needed in the future? e.g: would the CCF be used for encoding cadence internal data (e.g: elaboration serialization, etc.)?

[SupunS]

Reopening the issue, since the removal of PathLink value was reverted: #2381.

This would still be a problem for CCF.

[turbolent]

I followed up with the DX team: The Emulator provides the functionality to export values from account storage, and the Playground API in turn encodes the values further to JSON-CDC.

That means that we'll want to keep the export functionality (interpreter.Value to cadence.Value), and the JSON-CDC encoding, but can remove import related code.

In addition, we should add support for account link values.

For CCF, we can just ignore links.

I'll open a PR for that