fix(core): fix casting symbol values to strings in the prop type check warning

[jansiegel]

If the default value of the prop in the prop definition doesn't match the type of the prop, it should throw a warning, but instead it throws a TypeError when the default value is defined as a Symbol, e.g.:

testProp: {
    type: Number,
    default: Symbol('foo')
};

expected: Invalid prop: type check failed for prop "testProp". Expected Number, got Symbol
currently: TypeError: Cannot convert a Symbol value to a number

This PR changes the symbol->string casting from "" + value to String(value), as the first approach doesn't work well with symbols.
Also, I've added an isSymbol check, so it doesn't try to read the "value" of the symbol in the message warning + tests.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[posva]

Thank you but there is already #10529