debugger: add string validation for watch(expr) of node inspect

[cola119]

watch(expr) command of node inspect can accept non-string values, which result in inconsistent bugs.
This PR added the string validation to prevent this issue.

Before

$ node inspect -e "console.log()"
debug> watch()
debug> watch(1)
debug> watchers
  0: undefined = '<Invalid parameters>'
  1: 1 = '<Invalid parameters>'

After

$ node inspect -e "console.log()"
debug> watch()
node:internal/validators:119
    throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
    ^

Uncaught:
TypeError [ERR_INVALID_ARG_TYPE]: The "expression" argument must be of type string. Received undefined