fix: boolean values serialize as empty objects in JSON output

[boolangery]

What Problem This Solves

Boolean values (true / false) in parsed Lua ASTs were serializing as empty objects {} in JSON output, making them indistinguishable from each other and from other node types. Additionally, Node.to_json() crashed when _tokens was not set, and the value filter incorrectly dropped falsy values like False and 0.

Why This Change Was Made

Three changes in astnodes.py:

1. TrueExpr now sets self.value = True, FalseExpr sets self.value = False — so the boolean value is preserved as a node attribute
2. Node.to_json() filter changed from and v to and v is not None and v != [] — preserves False, 0, and other falsy-but-meaningful values
3. Node.to_json() now uses hasattr(self, "_tokens") guard instead of crashing

User Impact

Before:

{comments: [], wrapped: false}  // is this true or false? Impossible to tell.

After:

{True: {value: true, start_char: 10, stop_char: 13, ...}}
{False: {value: false, start_char: 26, stop_char: 30, ...}}

Evidence

• All 147 tests pass
• Updated test_to_pretty_json to match the corrected (and richer) output format

Fixes #76

[boolangery]

Please add an integration test

[boolangery]

Done — added test_cont_int_15 in test_integration.py (d67daff):

• Parses local a, b = true, false
• Asserts JSON contains "value": true and "value": false
• Asserts round-trip to_lua_source preserves boolean values

Full suite: 148/148 pass ✅