refactor: remove site config from theme config

[SukkaW]

What does it do?

# _config.yml under hexo directory
a:
  foo: 'bar'
c: 'vaz'

# _config.yml under theme directory
a:
  foo: 'abc'
b:
  baz: 'vrf'

Before this PR

In theme template:

• theme.b.baz --- vrf

• config.a.foo --- bar

• theme.a.foo --- abc

• config.c --- vaz

• theme.c --- vaz

After this PR

In theme template:

• theme.b.baz --- vrf

• config.a.foo --- bar

• theme.a.foo --- abc

• config.c --- vaz

• theme.c --- Throw undefined error

---

Since current behavior (merge site config into theme config) is not documented, I am assuming there is no theme developer is relying this behavior.

How to test

git clone -b remove-site-config-from-theme-config https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[curbengh]

Before deepMerge is used, it was Object.assign(), but I'm not sure why it's there in the first place. Duplicating the config into theme is indeed unnecessary. Another guess is that theme_config can override config, but this is undocumented and risky.