feat(theme-check-common): warn when select default isn't a valid option

[SinhSinhAn]

Summary

Closes #943.

Hey yall this PR wires up the warning @charlespwd asked for. Today you can happily write a select setting whose default doesn't exist in options, and theme-check says nothing. Same story for presets[].settings and a section's default.settings block — you only find out the value is bogus when the editor silently falls back or when QA catches it.

This PR adds a new check, ValidSelectDefault, that catches all three cases up front:

• If a select or radio setting's default isn't one of the options[].values, you get a warning on the default line with the list of values it should have been.
• If a preset sets a value for a select/radio setting that isn't in that setting's options, you get a warning right on the offending value inside presets[i].settings.
• Section schemas are covered the same way for their default.settings block.

It runs in two places, mirroring how ValidVisibleIf does it:

• LiquidCheck → {% schema %} in sections/*.liquid and blocks/*.liquid
• JSONCheck → config/settings_schema.json

The check is WARNING severity and recommended: true, so it shows up out of the box without being a merge-blocker for anyone with legacy schemas.

Example (from the issue)

{
  "type": "select",
  "id": "alignment",
  "default": "this-is-not-an-option",
  "options": [
    { "value": "flex-start", "label": "t:options.alignment.left" },
    { "value": "center",     "label": "t:options.alignment.center" },
    { "value": "flex-end",   "label": "t:options.alignment.right" }
  ]
}

Now flags: Default value "this-is-not-an-option" for setting "alignment" is not one of the valid options: "flex-start", "center", "flex-end".

And the preset case from the issue:

"presets": [
  {
    "name": "t:general.column",
    "settings": { "alignment": "this-is-not-an-option" }
  }
]

…gets a warning on the value itself, pointing right at the bad literal.

Test plan

• 13 new unit tests in valid-select-default/index.spec.ts covering:
  • valid + invalid default for select and radio
  • setting with no default (no-op)
  • non-choice settings left alone
  • valid + invalid preset values
  • preset referencing non-select setting (no-op)
  • section default.settings invalid value
  • config/settings_schema.json via JSONCheck (valid, invalid, wrong-file path)
  • combined default + preset case reports both
• Full @shopify/theme-check-common suite stays green (875 tests, 98 files)
• Changeset added as a minor bump