Add support for "_" special variable in REPL mode to store last expression value

[vt128]

This Pull Request implements the feature request to support the _ special variable in REPL mode, which stores the value of the last evaluated expression. This enhancement aligns the Starlark REPL behavior more closely with Python's REPL, improving the interactive experience for users. Fixed issue #569.

Changes Made:

• In repl.go, after each successful expression evaluation, the result is stored in the _ variable within the globals dictionary.
• The _ variable is only updated when an expression is evaluated, not after executing statements.
• If an error occurs during expression evaluation, the _ variable remains unchanged.
• No changes are made to script execution outside of REPL mode.

Benefits:

• Improves interactive debugging and experimentation workflow.
• Provides consistency with Python's REPL behavior, making it more intuitive for Python developers.
• Allows users to easily access the result of the last expression without explicitly assigning it to a variable.

Example Usage:

>>> 2 + 3
5
>>> _
5
>>> _ * 2
10
>>> _
10

Testing:

• Verified that the _ variable correctly stores the value of the last evaluated expression.
• Ensured that _ remains unchanged after executing statements or if an error occurs during evaluation.
• Confirmed that accessing _ before any expression has been evaluated results in a NameError, consistent with Python's REPL.

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[adonovan]

LGTM. Thanks!