test: add shebang regression tests and move changelog to Syntaxes section

Add extensionless regression test files for tclsh, wish, and expect
shebangs so syntect bumps don't silently break first-line detection.
Move changelog entry from Features to Syntaxes per reviewer suggestion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/examples/regression_tests/issue_3647_expect

@@ -0,0 +1,2 @@
+#!/usr/bin/expect -f
+set timeout 30

tests/examples/regression_tests/issue_3647_tclsh

@@ -0,0 +1,2 @@
+#!/usr/bin/env tclsh
+puts "Hello from tclsh"

tests/examples/regression_tests/issue_3647_wish

@@ -0,0 +1,2 @@
+#!/usr/bin/wish
+button .b -text "Click"