Update normalisation of Gmail messages

__tests__/files/gmail-gmail/chrome-chrome/02/received.eml

__tests__/files/gmail-gmail/chrome-chrome/02/sent.eml

__tests__/files/gmail-gmail/chrome-chrome/10/received.eml

__tests__/files/gmail-gmail/chrome-chrome/10/sent.eml

__tests__/helpers/index.ts

__tests__/html-gmail-gmail.test.ts

__tests__/htmltext-gmail-gmail.test.ts

__tests__/plain-gmail-gmail.test.ts

package.json

src/HTMLNormalizer/HTMLNormalizer.ts

src/HTMLNormalizer/strategies/common.ts

src/HTMLNormalizer/strategies/gmail.ts

src/PlainNormalizer/PlainNormalizer.ts

yarn.lock