API: ExtensionDtype._is_numeric
closes #22290 

split from https://github.com/pandas-dev/pandas/pull/22325

It's not clear what else we should be testing, since I'm not sure what all uses `Block.is_numeric`.

cc @jschendel 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22345?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b6e35ff`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22345/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22345?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22345   +/-   ##
=========================================
  Coverage          ?   92.05%           
=========================================
  Files             ?      169           
  Lines             ?    50715           
  Branches          ?        0           
=========================================
  Hits              ?    46685           
  Misses            ?     4030           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <100%> (?)` | |
| #single | `42.25% <50%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22345?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22345/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Jhc2UucHk=) | `92.68% <100%> (ø)` | |
| [pandas/core/arrays/integer.py](https://codecov.io/gh/pandas-dev/pandas/pull/22345/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2ludGVnZXIucHk=) | `94.71% <100%> (ø)` | |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/22345/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `93.84% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22345?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/22345?src=pr&el=footer). Last update [b6e35ff...e813855](https://codecov.io/gh/pandas-dev/pandas/pull/22345?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @jorisvandenbossche not sure if you comments are resolved? if so, pls merge away. @jorisvandenbossche all good? Thanks!