Parser improvments: quoted heredocs and short-hand interpolations

[aibaars]

I auto-formatted src/scanner.cc in 293179e?w=1 . It's probably easiest to review on a commit-by-commit basis.

This pull request changes the scanner to no longer recognize interpolations and escape sequences in single quoted heredocs:

<<'EOF'
This is \not\ an interpolation: #{foo}
EOF

In addition it adds support for "short-hand" string interpolations of global, class, and instance variables, e.g:

"value: #@value"

Checklist:

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[hendrikvanantwerpen]

Looks good! Just two small suggestions below.