[Data Mapper] Allow editing the whole let clause/let expression via Statement Editor

[madushajg]

Description:
Currently the DM only allows editing the expression part of let clause/let expression via the SE as there were no support to partially parse the intermediate clauses and let expressions.

But with ballerina-platform/ballerina-lang#37749 (released with 2201.3.0) and ballerina-platform/ballerina-lang#39215 (released with 2201.4.0) the partial parsing support is available.

Hence the DM should be able to render the SE for let clause/let expression with the support of above parsers.