Proof read toh-1 to -6

[chalin]

Proof reading TS and Dart tutorial: prose, code

• toh-1 - docs(toh-1/dart): minor edits #1614, docs(toh-1/ts): minor edits #1615.
• toh-2 - docs(toh-2/dart): copyedits #1621, docs(toh-2/ts): copyedits #1622.
• toh-3 - docs(toh-3/dart): copyedits and new snake_case glossary entry #1629, docs(toh-3/ts): copyedits #1630.
• toh-4 - docs(toh-4/dart): copyedits #1637, docs(toh-4/ts): copyedits #1638.
• toh-5 - docs(toh-5/dart): review edits and code cleanup #1643, docs(toh-5/ts): minor edits #1644.
• toh-6 - docs(toh-6/ts): minor edits and enhancements #1686 (ts). New Dart prose and coded are added via Create Dart toh-pt6, prose and code #1595.

[wardbell]

closed by PRs

[chalin]

Extending this issue to also cover toh-6 proof read.