Improve parser error handling

[DylanSp]

Two major areas of potential improvement:

• catching multiple parse errors and reporting all of them at once
• Augment parse error reports with source code location (filename, line #, column #)

Don't try to proceed past parsing if there are parse errors.

[DylanSp]

Parse error handling with panic/synchronization:

• https://craftinginterpreters.com/parsing-expressions.html#panic-mode-error-recovery
• https://craftinginterpreters.com/statements-and-state.html#parsing-variables