Fix: fix plugin-manager dispose method.

[liaodalin19903]

In the packages/designer/src/plugin/plugin-manager.ts's method dispose, the pluginContextMap and pluginPreference were not cleared.

Therefore, I'm submitting a Pull Request (PR) to fix this issue.

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11 out of 13 committers have signed the CLA.

:white_check_mark: JackLian
:white_check_mark: Osub
:white_check_mark: rainke
:white_check_mark: liujuping
:white_check_mark: zzzhrookie
:white_check_mark: DiscoverForever
:white_check_mark: woshilaoge
:white_check_mark: 1ncounter
:white_check_mark: liukairui
:white_check_mark: AndyJinSS
:white_check_mark: liaodalin19903
:x: actions-user
:x: 唐澜

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