Allow plugins to do range formatting

[dangmai]

Environments:

• Prettier Version: 1.19.1
• Running Prettier via: CLI
• Runtime: Node.js v12
• Operating System: Linux

Steps to reproduce:

Currently a Prettier plugin cannot do range formatting correctly, because Prettier core hard codes a list of parsers for isSourceElement here, which is used as a predicate for finding the start and end nodes that contain the range - see here.

Expected behavior:

Prettier should let its plugins set their own isSourceElement or the predicate, so that range formatting can be done by plugins.

Actual behavior:

The current predicate always returns false, which means that range formatting does not happen for custom parsers.

I can open a PR to add this functionality, but I want to check if there's a better solution than I'm not aware of before doing so.

Thank you!

[alexander-akait]

I think it is good idea, feel free to send a PR

[BoussonKarel]

I use Format selection a lot in typescript and handlebars files.

In next-gen Ember, we now have a new component authoring format called with "template tags". These are written in .gts and .gjs files, so we use a plugin (https://github.com/gitKrystan/prettier-plugin-ember-template-tag) to format these.

It would be really handy if I could use this Format selection.