Refactor enum values types

[elchininet]

The enum values types were using the enums' keys just because the keys and the values were the same, but it is better to get the string representation of the enums that returns the exact values.

Enum

enum Mode {
    combined = 'combined',
    override = 'override',
    diff = 'diff'
}

Enum values

// Before
type ModeValues = keyof typeof Mode; // 'combined' | 'override' | 'diff'

// After
type ModeValues = `${Mode}`; // 'combined' | 'override' | 'diff'

[coveralls]

coverage: 100.0%. remained the same
when pulling 753cb51 on change_values_types
into bb59cce on master.