Force color output in rescript_bsb.js if NINJA_ANSI_FORCED=1

[mediremi]

When running rescript build as a child process, color output can be enabled with NINJA_ANSI_FORCED=1. However, rescript_bsb.js does not respect this environmental variable and instead decides whether to colorize its log output based on process.stdout.isTTY and process.stderr.isTTY.

This PR fixes this issue by making rescript_bsb.js colorize its stdout and stderr log output if NINJA_ANSI_FORCED=1.

[cknitt]

Thank you for this PR! The changes look good to me. 👍

Probably out of scope for this PR, but I think it would make more sense from a user experience point of view to set a variable called something like RESCRIPT_FORCE_COLOR (or a similar command line flag) rather than NINJA_ANSI_FORCED. Especially as we are planning to move away from the ninja-based build system to rewatch.

[mediremi]

Yup NINJA_ANSI_FORCED isn't the clearest name for this env var. What do you think about FORCE_COLOR, without the RESCRIPT_ prefix? It's what node.js and chalk use.

I've created a draft PR using FORCE_COLOR (but happy to use RESCRIPT_FORCE_COLOR instead) that would supersede this one: #7033

I've the approach in that PR seems good I'll close this one and continue work there 👍