部分实现$random (#20)

[ifrvn]

#20

支持了SQLite和MySQL，可以使用.orderBy(() => $.random())来让数据库随机排序了。不接受参数。

core里的Eval.*** = 部分不是很理解，是在memory里用的？不确定是不是这么写

mongo不太熟，不会改...

[codecov]

Codecov Report

Merging #24 (d24c21d) into master (fbb2248) will decrease coverage by 0.04%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   97.83%   97.80%   -0.04%     
==========================================
  Files          23       23              
  Lines        4525     4548      +23     
  Branches     1068     1093      +25     
==========================================
+ Hits         4427     4448      +21     
- Misses         98      100       +2     

Impacted Files	Coverage Δ
packages/sql-utils/src/index.ts	97.46% <85.71%> (-0.43%)	⬇️
packages/core/src/eval.ts	94.53% <100.00%> (+0.14%)	⬆️
packages/mysql/src/index.ts	97.66% <100.00%> (+<0.01%)	⬆️
packages/sqlite/src/index.ts	96.81% <100.00%> (+<0.01%)	⬆️

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