refactor: Abstract executable detection and invocation pattern

[BYK]

Summary

The pattern of detecting an executable and invoking it with commands is repeated across many targets. This should be abstracted into a helper utility.

Current Pattern

const executable = await findExecutable(BIN, customBin);
if (!executable) {
  throw new ConfigurationError(`Cannot find '${BIN}' for publishing.`);
}
await spawnProcess(executable, [...args]);

Proposed Solution

Create a helper like:

interface ExecutableConfig {
  binName: string;
  customBinEnvVar?: string;
  errorContext: string;
}

async function runWithExecutable(
  config: ExecutableConfig,
  args: string[],
  options?: SpawnOptions
): Promise<Buffer>;

Affected Targets

• npm
• pypi
• crates
• gem
• pub-dev
• hex
• nuget
• Others

Related

Spawned from PR #707 review comment.

[BYK]

Implemented in PR #707 (commit 9ad56d0)