fix(typescript-estree): improve error messaging around project files

[bradzacher]

Fixes #853

People were having some problems because we now throw errors if files aren't included in the tsconfig.

This PR:

• Improves the error messages so that required steps to fix the issue are clearer
• Ensures that allowJs/checkJs are forced on, so that users don't ever have to manually turn it on in their configs.
  • This helps the system to "just work" without any real downside.
• Added tests to make sure the errors occur as expected.
• Improved documentation for the parser to clarify the setup for mixed js/ts codebases.

[JamesHenry]

LGTM, thanks!

[JamesHenry]

D'oh, should have checked the statuses first, tests are failing!

[codecov]

Codecov Report

Merging #866 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #866      +/-   ##
=========================================
- Coverage   94.17%   94.1%   -0.07%     
=========================================
  Files         113     113              
  Lines        4891    4906      +15     
  Branches     1362    1366       +4     
=========================================
+ Hits         4606    4617      +11     
- Misses        163     165       +2     
- Partials      122     124       +2

Impacted Files	Coverage Δ
packages/typescript-estree/src/tsconfig-parser.ts	88% <100%> (ø)	⬆️
packages/typescript-estree/src/convert.ts	98.96% <100%> (ø)	⬆️
packages/typescript-estree/src/parser.ts	94.24% <100%> (+0.69%)	⬆️
...-plugin/src/rules/no-unnecessary-type-assertion.ts	88.15% <0%> (-2.64%)	⬇️
packages/eslint-plugin/src/util/types.ts	79.48% <0%> (-2.57%)	⬇️