feat: Add `previousSeasons` to `BaseLeague` object

[DesiPilla]

Add a list of all previous seasons for a league. I know this is accurate for football leagues, and I assume(?) this is the same across all ESPN leagues and should be a part of the BaseLeague object.

[codecov-commenter]

Codecov Report

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

Comparison is base (cff22b5) 80.57% compared to head (8f9a834) 80.58%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #509 +/- ## ======================================= Coverage 80.57% 80.58% ======================================= Files 59 59 Lines 2111 2112 +1 ======================================= + Hits 1701 1702 +1 Misses 410 410 ```

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[DesiPilla]

An interesting note about ESPN's API on this matter:

Say you have a league that has been around since 2017.

• For currently active leagues (2023 right now), the previousSeasons will return the expected result: [2017, 2018, 2019, 202, 2021, 2022], all the years prior to the current year.
• For the most recent completed season (2022 in this case), the previousSeasons will return the expected result: [2017, 2018, 2019, 2020, 2021], all the years prior to the given league year.
• However, for all other completed years, the same result is returned. For example, even if you query for the year 2018, you would expect to get [2017], but the API will return [2017, 2018, 2019, 2020, 2021] instead.

[DesiPilla]

Updated the PR to only include seasons prior to the one being queried.

[DesiPilla]

@cwendt94 this PR is ready to be merged.