feat: Add flags to skip prompts and avoid modifying PATH

[nathanwhit]

Closes #297. Closes denoland/deno#25931

Adds a flag to skip prompts and accept defaults, and a flag to disable modifying the PATH. This is essentially the same as what rustup has for its installer.

Options:
  -y, --yes
    Skip interactive prompts and accept defaults
  --no-modify-path
    Don't add deno to the PATH environment variable
  -h, --help
    Print help

Also adds a couple of integration tests for the installer, simulating a PTY to check how it would behave in an actual terminal.

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The diff looks big, but the core of it is in install.sh and main.ts. The test file is mostly helper/boilerplate for the initial tests.

[bartlomieju]

This is great, nice work 👍