Allow a default dialect at the semantic model level

[fanruan-tuco]

Problem

Most models use the same dialect for nearly every field and metric, so this gets repeated throughout the file:

expression:
  dialects:
    - dialect: ANSI_SQL
      expression: SUM(orders.amount)

The repetition makes otherwise simple models harder to read and maintain.

Proposal

Allow a semantic model to declare a default dialect and use a string shorthand for expressions:

semantic_model:
  - name: sales
    default_dialect: ANSI_SQL
    datasets:
      - name: orders
        source: analytics.orders
        fields:
          - name: amount
            expression: amount
    metrics:
      - name: revenue
        expression: SUM(orders.amount)

Explicit dialects would still be supported when an expression needs an override:

expression:
  dialects:
    - dialect: ANSI_SQL
      expression: DATE_TRUNC('month', orders.created_at)
    - dialect: SNOWFLAKE
      expression: DATE_TRUNC('MONTH', orders.created_at)

Existing files would remain valid unchanged. The shorthand would only be allowed when default_dialect is present.

This is related to #52, but takes a smaller, backward-compatible approach: it keeps per-expression dialect overrides and scopes the default to each semantic model.