fix: allow usually-operator chars in regex parens

[reubeno]

In an extended test command, the following example regex is not allowed:

[[ str =~ < ]]

But if it's placed in parentheses, it is allowed:

[[ str =~ (<) ]]

These changes relax our parser's restrictions on which tokens typically parsed as operators may be allowed in that context. We add a compat test case to confirm.

Addresses one of the issues identified by #222.

[github-actions]

Test Results

  2 files    9 suites   1m 15s ⏱️
482 tests 482 ✅ 0 💤 0 ❌
955 runs  955 ✅ 0 💤 0 ❌

Results for commit 718403b.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.42 μs	3.42 μs	0.00 μs	⚪ Unchanged
instantiate_shell	59.81 μs	59.79 μs	-0.02 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	30321.58 μs	30446.13 μs	124.55 μs	⚪ Unchanged
parse_bash_completion	2782.41 μs	2764.22 μs	-18.20 μs	⚪ Unchanged
parse_sample_script	4.16 μs	4.18 μs	0.02 μs	⚪ Unchanged
run_echo_builtin_command	89.84 μs	89.22 μs	-0.62 μs	⚪ Unchanged
run_one_builtin_command	108.61 μs	109.06 μs	0.45 μs	⚪ Unchanged
run_one_external_command	1921.22 μs	1918.21 μs	-3.01 μs	⚪ Unchanged
run_one_external_command_directly	1013.11 μs	1006.29 μs	-6.82 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/jobs.rs	🔴 42.42%	🔴 37.23%	🔴 -5.19%
Overall Coverage	🟢 76.42%	🟢 76.34%	🔴 -0.08%

Minimum allowed coverage is 70%, this run produced 76.34%