dbchecker: fails on json format links

The db checker fails to parse links like
```
field(INPC, {const: "$(TYPE)"})
```
this is a valid epics 7 json format initialisation, it was used in this case as by default a string is taken as a PV link and instead a constant value was needed, it could be done by linking to an additional PV but the json link is more compact

## Acceptance criteria

dbChecker understands json links as per https://epics.anl.gov/base/R7-0/1-docs/links.html