fix: validate sources value to be string

[danielroe]

Currently using a package that injects an object as theme in nuxt.config and is broken since nuxt/framework#7131. Obviously that will need to be fixed in the theme, but in a similar case in future it will be unclear what the issue is because it triggers the following error:

Nuxi 3.0.0-rc.10-27706138.b31405f
Nuxt 3.0.0-rc.10-27706138.b31405f with Nitro 0.5.1-27706249.08ac81b

 ERROR  Cannot start nuxt:  source.startsWith is not a function

  at node_modules/.pnpm/c12@0.2.10/node_modules/c12/dist/index.mjs:174:44
  at Array.some (<anonymous>)
  at resolveConfig (node_modules/.pnpm/c12@0.2.10/node_modules/c12/dist/index.mjs:174:20)
  at extendConfig (node_modules/.pnpm/c12@0.2.10/node_modules/c12/dist/index.mjs:151:27)
  at extendConfig (node_modules/.pnpm/c12@0.2.10/node_modules/c12/dist/index.mjs:156:11)
  at async loadConfig (node_modules/.pnpm/c12@0.2.10/node_modules/c12/dist/index.mjs:114:5)

This change makes it warn instead:

 WARN  Cannot extend config from {"meta":{"name":"Nuxt Elements","description":"Building blocks for your next application.","author":"NuxtLabs"}} in /private/tmp/studio/node_modules/.pnpm/@nuxt-themes+elements-edge@0.0.1-fdfc0b1/node_modules/@nuxt-themes/elements-edge/theme

It would also be equally easy to filter it out at L136 but thought you might prefer the warning for now.

[codecov]

Codecov Report

Merging #32 (7ee2658) into main (faa482d) will decrease coverage by 1.31%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   76.38%   75.07%   -1.32%     
==========================================
  Files           3        3              
  Lines         343      349       +6     
  Branches       46       47       +1     
==========================================
  Hits          262      262              
- Misses         36       41       +5     
- Partials       45       46       +1     

Impacted Files	Coverage Δ
src/loader.ts	77.57% <0.00%> (-2.24%)	⬇️

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