api: move `filter` from `CompliantSeries` to `CompliantExpr`

[MarcoGorelli]

Splitting this off from #3152, where I noticed it

Although we don't currently implement filter for lazy expressions, that's just an implementation detail, and it should be possible to support filter if the expression results in a scalar-like value

For example:

• nw.col('a').filter(nw.col('b')>1).sum() should be valid lazy + eager
• nw.col('a').filter(nw.col('b')>1) should be eager-only