Respect NO_COLOR and non-TTY output #27

Context

src/becwright/cli.py defines ANSI color constants unconditionally (RED = "\033[91m", etc.) and always emits them. When becwright check output is piped to a file or another tool, raw escape codes leak in, and the NO_COLOR convention is ignored. (The --json output is unaffected — this is about the human-readable output.)

Proposal

Disable colors when either:

• the NO_COLOR environment variable is set, or
• stdout is not a TTY (sys.stdout.isatty() is false).

Optionally add an explicit --no-color flag. Simplest implementation: set the color constants to empty strings in those cases.

Acceptance criteria

• becwright check | cat and NO_COLOR=1 becwright check emit no escape sequences.
• A test asserts this (e.g. capsys + monkeypatching isatty).

Files: src/becwright/cli.py (color constants near the top, _print_result).