gh-35517: implement the depth of a quasimodular form
<!-- Please provide a concise, informative and self-explanatory title.
-->
<!-- Don't put issue numbers in the title. Put it in the Description
below. -->
<!-- For example, instead of "Fixes #12345", use "Add a new method to
multiply two integers" -->
### :books: Description
<!-- Describe your changes here in detail. -->
<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->
This PR implements the depth of a quasimodular form. Recall that the
depth of a quasimodular form $f$ of weight $k$ is the integer $p$ such
that:
$$
f = f_0 + f_1 E_2 + \cdots + f_p E_2^p
$$
where $f_i$ is of weight $k - 2*i$ and $f_p\neq 0$.
CC: @videlec
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->
- [ x] The title is concise, informative, and self-explanatory.
- [x ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion. (not applicable)
- [x ] I have created tests covering the changes.
- [x ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/35517
Reported by: David Ayotte
Reviewer(s): David Ayotte, grhkm21
