Fix showIndex() query regexp

[nunofgs]

Pull Request check-list

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Have you added new tests to prevent regressions?

Description of change

Fixes an issue when calling showIndex() on an index that contains a newline in its "fields". Example:

CREATE INDEX "foobar" ON "users" USING btree ((
CASE (settings->>'type')
    WHEN 'foo' THEN 'biz'
    WHEN 'bar' THEN 'baz'
    ELSE 'qux'
END)) WITH (fillfactor='70')

The issue is with a regexp that is matching against .* (which doesn't match newlines). This PR replaces that expression with [^]* which matches any character that is not nothing.

The error that was previously seen when a newline was present was:

TypeError: Cannot read property '1' of null
  at node_modules/sequelize/lib/dialects/postgres/query.js:139:85
  at Array.forEach (native)
  at node_modules/sequelize/lib/dialects/postgres/query.js:135:15

[sushantdhiman]

@nunofgs Needs changelog entry as well

[codecov-io]

Codecov Report

Merging #7492 into master will not change coverage.
The diff coverage is 100%.

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