This required 2 changes:
1. In the parser, create a new variable scope, so the variable is
declared in it instead of the scope in which the 'for' is found.
2. On execute, push the variable into the newly created block. Existing
code created an empty block (no variables, no arguments) which
allows Interpreter::enter_scope() to skip the creation of a new
environment, therefore when the variable initializer is executed, it
sets the variable to the outer scope. By attaching the variable to
the new block, the block gets a new environment.
This is only needed for 'let' / 'const' declarations, since 'var'
declarations are expected to leak.
Fixes: #2103
