Change Details

[Part of ENG-843](https://linear.app/comm/issue/ENG-843) Introduces `plain` `Button` variant with minimal css properties. It becomes the default variant, so this diff also changes any `Button`s without explicit variant to use `filled`. This variant is used in a later diff.

[Part of ENG-843](https://linear.app/comm/issue/ENG-843) Introduces `plain` `Button` variant with minimal css properties. It becomes the default variant, so this diff also changes any `Button`s without explicit variant to use `filled`. Also makes the `onClick` nullable and sets the `type` to `button` to make it more general. This variant is used in a later diff.

[Part of ENG-843](https://linear.app/comm/issue/ENG-843) Introduces `plain` `Button` variant with minimal css properties. It becomes the default variant, so this diff also changes any `Button`s without explicit variant to use `filled`. Also makes the `onClick` nullable and sets the `type` to `button` to make it more general. This variant is used in a later diff.