Add test and followup for `StyledStringsExt`

[kimikage]

I do not understand the need for me to create this PR, but this PR itself might be necessary for sound development.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.64%. Comparing base (142353d) to head (cdd28e1). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #302 +/- ## ========================================== - Coverage 84.31% 83.64% -0.68% ========================================== Files 8 9 +1 Lines 778 746 -32 ========================================== - Hits 656 624 -32 Misses 122 122 ```

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[kimikage]

@tecosaur Is the following intended behavior?

julia> StyledStrings.SimpleColor(RGB(1, 0, 0))
ERROR: MethodError: Cannot `convert` an object of type
  RGB{FixedPointNumbers.N0f8} to an object of type
  Union{@NamedTuple{r::UInt8, g::UInt8, b::UInt8}, Symbol}
The function `convert` exists, but no method is defined for this combination of argument types.

[tecosaur]

@tecosaur Is the following intended behavior?

julia> StyledStrings.SimpleColor(RGB(1, 0, 0))
ERROR: MethodError: Cannot `convert` an object of type
  RGB{FixedPointNumbers.N0f8} to an object of type
  Union{@NamedTuple{r::UInt8, g::UInt8, b::UInt8}, Symbol}
The function `convert` exists, but no method is defined for this combination of argument types.

I'd say so, given I didn't intend for the constructor to work directly with Colorants regardless of the conversion methods implemented.

[kimikage]

I didn't intend for the constructor to work directly with Colorants

If you do not plan to make any changes in this package, I am ok with it. Now let's merge this.