Avoid usage of "Feature Flag" unless we are talking specifically about
the response from github features api. Otherwise, use terms like
"Toggleable features".
Note both "toggleable" and "togglable" appear to be valid spellings of
the word. I chose the first for no good reason.
This commit centralizes how feature flags are handled. All feature flags
must now add an entry in the `featureFlagConfig` dictionary. This
dictionary associates the flag with an environment variable name and
optionally a minimum version for CodeQL.
The new logic is:
- if the environment variable is set to false: disabled
- if the minimum version requirement specified and met: disabled
- if the environment variable is set to true: enable
- Otherwise check feature flag enablement from the server
