fix(params): strip $action as a whole token, not a substring

[tiagolauer]

Fixes #298.

The bug

const db = createTypedDb<DB, { placeholders: 'question' }>(executor);
db.query('select id from users where kind = $actionType', 'x'); // compiles

UsedPlaceholderStyles came back never for that query, so the brand check passed for any dialect, while Params still assigned the token a [string] slot. A mysql or sqlite driver can never bind a $name, so this is exactly the guaranteed runtime failure the brand exists to prevent. Same for $action_id, $actionable, and friends.

The fix

The two layers disagreed about what $action means. StripDollarAction removed the substring anywhere, so $actiontype became type before the dollar scan ran; IsPlaceholder excludes only the exact token. The strip now follows IsPlaceholder's rule: keep $action when an identifier character follows it.

MERGE is unaffected — a real output $action, inserted.id has a comma after it, and tests/dialect-mssql.test-d.ts pins that (the pseudo-column takes no parameter slot and does not make the statement look dollar-styled).

Verification

tests/dialect-brand.test-d.ts, four new cases. The three @ts-expect-error ones are red on master, which tsc reports precisely because the directive is unused there:

tests/dialect-brand.test-d.ts(58,3): error TS2578: Unused '@ts-expect-error' directive.
tests/dialect-brand.test-d.ts(61,3): error TS2578: Unused '@ts-expect-error' directive.
tests/dialect-brand.test-d.ts(64,3): error TS2578: Unused '@ts-expect-error' directive.

• a question-style client rejects $actionType, $action_id and $actionable
• a control: a dollar-style client still accepts $actionType

tsc --noEmit clean, budget 192,568 (+82).