Add encoder formatting options: integerGrouping, trailingComma, dottedKeys

[dereuromark]

Summary

Adds new EncoderOptions for controlling output formatting:

• integerGrouping: Format large integers with underscores for readability (1_000_000 instead of 1000000)
• trailingComma: Add trailing commas to arrays ([1, 2, 3,]) - useful for cleaner git diffs
• dottedKeys: Use dotted key syntax instead of table headers (a.b.c = 1 instead of [a.b]\nc = 1)

All options default to false, preserving existing behavior.

Usage

$encoder = new Encoder(new EncoderOptions(
    integerGrouping: true,
    trailingComma: true,
    dottedKeys: true,
));

Notes

• dottedKeys still uses [[table]] syntax for arrays of tables (dotted keys can't represent them)
• All new options are thoroughly tested