fix: ignore "package.json" as config file when it's invalid JSON

[iiroj]

This PR hides the JSON parser error when trying to load lint-staged config from an invalid package.json file:

% node bin/lint-staged.js
SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at jsonParse (file:///Users/iiro/git/lint-staged/lib/loadConfig.js:30:43)
    at Object.load (/Users/iiro/git/lint-staged/node_modules/lilconfig/dist/index.js:148:35)
    at async loadConfig (file:///Users/iiro/git/lint-staged/lib/loadConfig.js:65:20)
    at async Promise.all (index 1)
    at async searchConfigs (file:///Users/iiro/git/lint-staged/lib/searchConfigs.js:84:3)
    at async runAll (file:///Users/iiro/git/lint-staged/lib/runAll.js:129:24)
    at async lintStaged (file:///Users/iiro/git/lint-staged/lib/index.js:108:17)
✔ Preparing lint-staged...
✔ Running tasks for staged files...
✔ Applying modifications from tasks...
✔ Cleaning up temporary files...

Instead a debug log entry will be emitted (in this case an empty file):

  lint-staged:loadConfig Ignoring invalid package file `/Users/iiro/git/lint-staged/test/package.json` with content:
  lint-staged:loadConfig  +0ms
  lint-staged:loadConfig Successfully loaded config from `/Users/iiro/git/lint-staged/test/package.json`:
  lint-staged:loadConfig null +0ms

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (05fb382) 100.00% compared to head (1b51eec) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1281   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          733       740    +7     
  Branches       197       198    +1     
=========================================
+ Hits           733       740    +7     

Impacted Files	Coverage Δ
lib/loadConfig.js	100.00% <100.00%> (ø)

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