Add a `blocklyFieldText` CSS class to fields' text elements

[BeksOmega]

In the field's createTextElement_ method, add a blocklyFieldText class to the class property.

https://github.com/google/blockly/blob/8323642d0eb677648f004f294208d2734c6e8d65/core/field.ts#L376

[ga-fleury]

Hey @BeksOmega, can you assign this to me?

[BeksOmega]

Closed by #8302