fix(context): let getters return literal instead of string type

[chentsulin]

This might improve some type checks.

[codecov-io]

Codecov Report

Merging #724 into release/1.4 will not change coverage by %.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           release/1.4     #724   +/-   ##
============================================
  Coverage        80.91%   80.91%           
============================================
  Files              108      108           
  Lines             4511     4511           
  Branches          1083     1127   +44     
============================================
  Hits              3650     3650           
+ Misses             815      808    -7     
- Partials            46       53    +7     

Impacted Files	Coverage Δ
packages/bottender/src/line/LineContext.ts	71.95% <ø> (ø)
...ckages/bottender/src/messenger/MessengerContext.ts	96.99% <ø> (ø)
packages/bottender/src/slack/SlackContext.ts	86.20% <ø> (ø)
packages/bottender/src/telegram/TelegramContext.ts	53.08% <ø> (ø)
packages/bottender/src/viber/ViberContext.ts	92.85% <ø> (ø)
packages/bottender/src/whatsapp/WhatsappContext.ts	83.33% <ø> (ø)
packages/bottender/src/chain.ts	76.92% <0.00%> (ø)
packages/bottender/src/getClient.ts	55.55% <0.00%> (ø)
packages/bottender/src/router/index.ts	90.66% <0.00%> (ø)
packages/create-bottender-app/src/shared/log.ts	0.00% <0.00%> (ø)

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