Add theme keys for the picker header area

[ofzo]

full fill column's background for picker, it can be defined use

"ui.picker.header" = { fg= "white", bg = "red" }

[the-mikedavis]

Using the ui.picker.header key will change the meaning for it that I had in mind when introducing it. The current behavior is that the bg or modifiers (underlined for example) will only affect the column names. For example in my theme I have "ui.picker.header" = { underline.style = "line" }:

But if we apply the style to the header area:

I could see both ways being useful so I think we should change the keys around so that both are possible. Let's use ui.picker.header for the header (Row)'s style and introduce ui.picker.header.column that replaces the current use of ui.picker.header. Then ui.picker.header.active can move to ui.picker.header.column.active

We usually try to avoid breaking changes but these theme keys were merged very recently and no themes are using it in the runtime/themes dir yet so I think it's fine in this case.

[ofzo]

@the-mikedavis I have resolve it.

[the-mikedavis]

Looks good codewise. We just need to update the docs about these keys too:

helix/book/src/themes.md

Lines 300 to 301 in 6eae846

	| `ui.picker.header` | Column names in pickers with multiple columns |
	| `ui.picker.header.active` | The column name in pickers with multiple columns where the cursor is entering into. |

[ofzo]

@the-mikedavis yes ，the document has been supplemented.