Flow: Backport lit-html to 2.5

[caalador]

Backport the lit html feature to 2.4.
Also remember taking in the documentation.

[Artur-]

Also note that LitTemplate is meant to use with TypeScript so you need to backport TypeScript support also

[pleku]

EDIT: TS support is not needed for _live reload_ for frontend changes.

Also note that LitTemplate is meant to use with TypeScript so you need to backport TypeScript support also

This is a bit controversial, since in 17 the basic lit support and designer support is based on ES modules instead of TS.
So basically lit support can be backported to 14 without TS support.

[jcgueriaud1]

Any idea if TS will be backported in flow 2.x with Lit template?
Lit can be backported without TS and TS can be backported without lit but the entire documentation to use lit in Vaadin is written in Typescript.

[pleku]

Any idea if TS will be backported in flow 2.x with Lit template?

As TS is what we recommend people to use in their lit templates, I'm fairly sure we'll backport both at the same time.

[mvysny]

@pleku what is the target Vaadin version for Lit and TS please? Vaadin 14.4?

[pleku]

@mvysny it is 14.5.

[pleku]

Acceptance Criteria

Lit-templates:

• [x] The basic lit template support and the reading attribute values should be backported (former in 4.0 and latter in 5.0)
• [x] It should be possible to use either TS (recommended) or JS for the lit templates, but the TS compilation should not cause a performance hit for apps that don't have any TS templates. The TS support needs to be backported if possible, but since the master branch awesome-ts-loader which is abandoned, we can use something else for 2.5 immediately
• [x] Documentation is updated for v14 branch of the new docs site (both lit + polymer), need to mention that the changes are from 14.5. The polymer part should mention that _it is recommended to use lit instead, but polymer is still supported and will be available in the nextLTS (after 14)_
• [x] SVC Java+HTML templates are changed to be lit template based (done for v18 version, backport if possible?)

Polymer-templates:

• [x] Not deprecated in 2.x series. But javadocs should be updated that LitTemplate is the recommended way.
• [x] Updating Bakery from 14.4 to 14.5 shouldn't require any changes to polymer templates
• In Flow 5.0 the polymer template parts were moved to another module, it is an open question if this would be done for 2.5 too. It should be 100% backwards compatible for application code if we do so - if someone is relying on some internal parts of the FW then they need to make the trivial changes.

Explicitly outside of the scope: Updating Bakery to be lit template based

[caalador]

SVC will need to delete the lts folders in resources to get LitTemplate version to LTS.
This shoud be done when 14.5 becomes the LTS version.