feat: style the braces of ${VAR} as part of the env var

[WynandVStaden]

Follow-up to #1030, as asked in #1030 (comment) by @MartenBE (issue #1029): colour the ${ and } of ${VAR} as part of the variable.

Change

• Parser (DParser): when ${ is followed by a plain variable name and then directly by }, the ${ and } tokens are marked is_env_var too, the same way the $ of $VAR already is.
• Styling (token_to_style): the env-var colour is now checked before the rainbow bracket colour, so those two tokens take the colour of the variable. They follow the name: recognised or unrecognised.

Input	${ and }
${HOME}, "${USER}"	env-var colour
${UNRECOGNISED}	unrecognised env-var colour
${ARR[0]}, ${HOME:-/tmp}, ${#PATH}	rainbow bracket colour, as before

In the last row the braces enclose more than the name, so I left them as delimiters. No other delimiter carries is_env_var, so the reordering in token_to_style only affects these two tokens. The highlight of a matching pair under the cursor still comes first.

Tests

• test_param_expansion_name_is_env_var now checks which delimiters are marked.
• test_brace_delimited_env_var_styling checks the colour of the delimiters for a recognised and an unrecognised variable.
• New test_brace_delimiters_around_more_than_a_name_stay_brackets for ${HOME:-/tmp}.

The first two fail with the new parser lines disabled.

Run on Ubuntu 24.04, rustc 1.99.0:

• cargo test --lib: 885 passed, 0 failed, 1 ignored (884 on master)
• cargo fmt --all -- --check: clean
• cargo clippy --workspace --all-features -- -D warnings: clean

Not done: the Docker integration tests, and I did not look at the colours in a terminal; the styling is covered by the unit tests only.

This is a choice about looks, so if you prefer the braces to stay rainbow-coloured, feel free to close it.