feat: environment variables turn a channel and its streaming on

[mekjr1]

Fixes #43.

A program that starts compa-kernel gateway could set the web chat's token with COMPA_CHANNELS_WEB_TOKEN, but no variable turned the channel on. So it had to write channel_list.web into config.json. Because a file entry replaces the default one whole, it also had to write streaming and the rest, and keep them in step with Compa's defaults.

Now:

• COMPA_CHANNELS_<NAME>_ENABLED sets a channel_list entry's enabled. <NAME> is the entry's name in capitals, with _ for each character other than a letter or digit, so COMPA_CHANNELS_WEB_ENABLED=true turns on the default web entry, streaming included.
  • It's read in initChannelList, next to the settings' env tags, so it applies to every entry, the defaults included.
  • A value that isn't a boolean fails the load, naming the variable and the channel, as a bad env-tag value does. An empty value counts as unset.
• COMPA_CHANNELS_WEB_STREAMING_ENABLED, _THROTTLE_SECONDS and _MIN_GROWTH_CHARS set the web chat's streaming. They work like Telegram's existing variables, which now share their code (applyStreamingEnvOverrides).
• Both are recorded as environment overrides, so SaveConfig writes the file's value back, as for the other variables.

docs/cli.md lists the variables. It also says that saving keeps a file's value for a setting a variable set, unless you change the setting. CHANGELOG line under Added.

Tests:

• TestChannelEnabledEnvOverride loads a config with the variables set. It checks that web turns on, Telegram turns off and web streaming turns off. After SaveConfig, the file keeps its own values for all three.
• That test fails when the variable isn't applied, when the override isn't recorded (for either channel), and without the web streaming variable.
• TestChannelEnabledEnvOverrideRejectsABadValue and TestChannelEnabledEnvName cover a bad value and the name mapping.

[mekjr1]

Superseded by #63, which includes this change or replaces it.