fix: generated self-referencing CSS variables

[wtto00]

Fix incorrect output like blow:

.dark {
    --un-preset-theme-colors-container: var(--un-preset-theme-colors-container);
    --un-preset-theme-colors-container--alpha: var(--un-preset-theme-colors-container--alpha, 1);
    --un-preset-theme-colors-border: var(--un-preset-theme-colors-border);
    --un-preset-theme-colors-border--alpha: var(--un-preset-theme-colors-border--alpha, 1);
    --un-preset-theme-colors-text: var(--un-preset-theme-colors-text);
    --un-preset-theme-colors-text--alpha: var(--un-preset-theme-colors-text--alpha, 1);
    --un-preset-theme-colors-primary: var(--un-preset-theme-colors-primary);
    --un-preset-theme-colors-primary--alpha: var(--un-preset-theme-colors-primary--alpha, 1);
    --un-preset-theme-colors-primary-border: var(--un-preset-theme-colors-primary-border);
    --un-preset-theme-colors-primary-border--alpha: var(--un-preset-theme-colors-primary-border--alpha, 1);
}

fix #81

TODO: Add tests.
The existing tests cannot identify this issue. I referred to https://github.com/unocss/unocss/blob/main/packages/runtime/test-dom/runtime-dom.test.ts and attempted the runtime-dom vitest tests, but they also failed to detect this problem.

Regarding the unit test, I wasn't able to find a better solution, so I had to resort to comparing the bundled CSS output.

My approach to fixing the issue is to check whether the CSS variable already exists when assigning it. If it does, the existing value will be used rather than generating a new one.

[Dunqing]

Thanks!!

[Dunqing]

I just found the test file is not in a suitable place where we should move to https://github.com/unpreset/unocss-preset-theme/tree/main/tests

[wtto00]

I just found the test file is not in a suitable place where we should move to https://github.com/unpreset/unocss-preset-theme/tree/main/tests

Another pr by me or push to main by you?