feat(eslint-plugin): added new rule prefer-readonly

[JoshuaKGoldberg]

Adds the equivalent of TSLint's prefer-readonly rule.

Edit: will fix up post merge soon...! ✔️

[codecov]

Codecov Report

Merging #555 into master will increase coverage by 0.07%.
The diff coverage is 97.43%.

@@            Coverage Diff            @@
##           master    #555      +/-   ##
=========================================
+ Coverage   94.22%   94.3%   +0.07%     
=========================================
  Files         108     109       +1     
  Lines        4419    4533     +114     
  Branches     1217    1253      +36     
=========================================
+ Hits         4164    4275     +111     
- Misses        147     148       +1     
- Partials      108     110       +2

Impacted Files	Coverage Δ
...n/src/rules/indent-new-do-not-use/OffsetStorage.ts	100% <ø> (ø)	⬆️
...lugin/src/rules/indent-new-do-not-use/TokenInfo.ts	100% <ø> (ø)	⬆️
packages/typescript-estree/src/convert.ts	98.96% <100%> (ø)	⬆️
packages/eslint-plugin/src/rules/index.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin-tslint/src/custom-linter.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin/src/util/types.ts	82.05% <83.33%> (+0.23%)	⬆️
...ackages/eslint-plugin/src/rules/prefer-readonly.ts	99% <99%> (ø)

[bradzacher]

great work!
code itself LGTM.
few nits