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

- Change env var name for `MlPoweredQueriesEnabled`
- Throw error if minimumVersion is specified, but CodeQL argument is not
  supplied.
- Fix failing tests. Note that I removed a config-utils test because it
  is no longer relevant since we handle codeql minimum versions in the
  `getValue` function.

Internal refactoring so that `GitHubFeatureFlags` is
private only. The public facing class is `Features`.

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.

…toring

…toring