deps: eslint v7, prettier v2

[chentsulin]

Upgrade ESLint to v7, Prettier to v2.

[codecov]

Codecov Report

Merging #589 into master will increase coverage by 0.27%.
The diff coverage is 89.72%.

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   94.15%   94.43%   +0.27%     
==========================================
  Files          25       25              
  Lines        1815     1796      -19     
  Branches      352      332      -20     
==========================================
- Hits         1709     1696      -13     
+ Misses        106      100       -6     

Impacted Files	Coverage Δ
packages/messaging-api-wechat/src/WechatClient.ts	97.40% <ø> (ø)
...ges/messaging-api-messenger/src/MessengerClient.ts	91.61% <82.89%> (+0.86%)	⬆️
packages/messaging-api-line/src/LineClient.ts	92.52% <96.49%> (ø)
packages/axios-error/src/index.ts	100.00% <100.00%> (ø)
packages/messaging-api-line/src/LineNotify.ts	82.69% <100.00%> (ø)
...ckages/messaging-api-slack/src/SlackOAuthClient.ts	92.68% <100.00%> (ø)
...ages/messaging-api-slack/src/SlackWebhookClient.ts	100.00% <100.00%> (ø)
...kages/messaging-api-telegram/src/TelegramClient.ts	99.44% <100.00%> (ø)

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