ENH: Number formatting support for excel styles
- [x] closes #22027 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Add number formatting support to the excel styles using a fake css entry. 

```css
number-format: 0%;
```

Added new tests, updated docs, and manually verified outputs using both excel output engines that support styling. 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=h1) Report
> Merging [#22015](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b97545563f724c55bc40271adab64e0239e65328?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22015/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22015      +/-   ##
==========================================
+ Coverage      92%      92%   +<.01%     
==========================================
  Files         168      168              
  Lines       50603    50605       +2     
==========================================
+ Hits        46555    46557       +2     
  Misses       4048     4048
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.4% <100%> (ø)` | :arrow_up: |
| #single | `42.19% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/22015/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.39% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=footer). Last update [b975455...4c28407](https://codecov.io/gh/pandas-dev/pandas/pull/22015?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Haven't reviewed code in detail yet but does this reference an existing issue? If not can you open and add reference in whatsnew? Done  @marknsikora : This looks good at the moment.  Could you add `whatsnew` entry for `0.24.0` ? Thanks @marknsikora .

