feat: support interpolating a CommandBuilder into a template literal expression

[dsherret]

Implements dsherret/dax#239.

Interpolating a CommandBuilder into a $ template literal now substitutes its captured stdout, similar to $(...) command substitution:

const cmd = $`echo 1`;
await $`echo ${cmd}`; // 1
await $`echo 'example ${cmd} example'`; // works inside quotes
await $`cat < ${cmd}`; // and as an input redirect

How it works

• Template layer: an interpolated builder is registered in a commandRefs map on the command state and a sentinel token (\0<nonce>:<id>\0) is emitted into the command text. The nonce is per-process random so escaped user text can't forge one, and NUL passes through the parser as literal text in bare, single-quoted, and double-quoted contexts.
• Post-parse rewrite: after parsing, replaceCommandRefSentinels walks the AST and splits sentinel-bearing text parts into a new commandRef word part (including inside quoted parts, braces, subshells, redirect words, and env var values). No changes to the Rust parser.
• Execution: when word evaluation reaches a commandRef, an invoker runs the builder with stdout piped. Evaluation is lazy and sequential, matching how words evaluate today — exit 1 && echo ${cmd} never runs cmd.

Semantics

• output is whitespace-normalized the same way as $(...)
• the builder executes with its own configuration (env, cwd, stdin, hooks); stdout is captured and an "inherit" stderr routes to the evaluating command's stderr
• a failing interpolated command fails the outer command with its exit code and a stderr message like failed evaluating interpolated command `exit 5`. Exited with code: 5 — add .noThrow() to the interpolated command to tolerate failure
• killing the outer command kills any in-flight interpolated command (linked through an intermediate KillController so kills only propagate outer → inner)
• runs once per interpolation site
• printCommand / tail headers display the sentinel as $(inner command text)

Also fixes a pre-existing gap where a ShellEvaluateError (e.g. failglob) thrown while evaluating a redirect word or a standalone VAR= assignment escaped the executor instead of becoming a command error, and gives ShellEvaluateError an exit code so interpolated command failures propagate theirs.