This implements brace-style env vars inside backticks with `echo` and
fixes termination of unbraced `$VAR`. Most importantly, variables can
now be adjacent to static text (e.g., `foo${ENV}bar`), which previously
did not work and caused confusion. This aligns behavior with common
bash expectations.
Why
- Users expect `${VAR}` and `$VAR!` to expand while keeping punctuation.
- Concatenations like `foo${ENV}bar` are common and should be valid.
What
- Add support for braced variables `${VAR}` with proper `}` handling.
- For unbraced `$VAR`, stop the name at the first non `[A-Za-z0-9_]`
char and emit that char (e.g., `!`, `.`) as literal output.
- Improve error handling for overlong env var names.
- Keep other shell features (e.g., `$(pwd)`) unsupported by design.
Docs
- Update `constant_strings.rst` to document `${VAR}`, the new
termination rules, and examples including `foo${ENV}bar` and `$VAR!`.
Tests
- Add `rscript_backticks_braces_envvar.sh` for `${VAR}` support.
- Add `rscript_backticks_static_text.sh` for `$VAR!` and adjacency.
- Convert `rscript_backticks_empty_envvar-vg.sh` to non-VG variant and
adjust `tests/Makefile.am`.
Compatibility
- Backward compatible. Changes affect only previously broken edge cases.
Misc
- Refresh lexer copyright years.
