debug: use arrow functions for lexical `this`

Checklist

• make -j4 test (UNIX) or vcbuild test nosign (Windows) passes
• the commit message follows commit guidelines

Affected core subsystem(s)

debug

( specifically: lib/_debug_agent.js )

Description of change

Use arrow functions to replace 3x var self = this; ... function() { self.whatever() } pattern.

See #7414 (closed) for more details.