feat(sources): allow setting kind_name and kind_icon in provider config

[andreihh]

Setting the kind_name and kind_icon on every item requires overriding the transform_items function, which can get tedious. Oftenly, custom sources provide only one item kind (e.g., blink-copilot, blink-emoji, blink-cmp-env, blink-ripgrep, css-vars.nvim, blink-cmp-spell).

Allow providers to specify a default kind_name and kind_icon to apply to all of its returned items. These should have lower priority than the kind_name and kind_icon set explicitly on returned items, but higher priority than globals (e.g., appearance.kind_icons). This allows providers to set finer-grained kind_name or kind_icon on an item-by-item basis, but also simplifies logic for providers with only one kind.

NOTE: I tested the changes locally and it seems to work.

[Kaiser-Yang]

There is no need to do this, actually with kind_name and kind_icon in blink completion item type. The provider can provide when generating the items, so I think it should be done by the providers' developers to add configuration for users to specify icons rather than the users. BTW, this configuration option can not work for multi types and icons for one provider, such as blink-cmp-git for open, closed, and merged prs.