Add 'package.json' templating

[vince-fugnitto]

Description

It'd be nice to utilize some form of templating for the various image's package.json in order to increase maintainability, and reduce duplication. This will make it much easier to customize the different images while keeping a standard base.

We could use a template/replacenent variable system to generate each application's package.json from an application-specific template. Replacement variables would be replaced by an arbitrary value for a given run, depending on some settings.

With such a mechanism, the resolutions block of each application's package.json could be defined in a single place and injected everywhere.

Something like this could help:
https://www.npmjs.com/package/envsub

Example

{
    "private": true,
    "theia": {
        "frontend": {
            "config": {
                "applicationName": "Theia Java Example",
                "preferences": {
                    %%PREFERENCES%%
                }
            }
        }
    },
    "dependencies": {
        %%DEPENDENCIES%%
        %%DEPENDENCIES_EXTRA%%
    },
    "resolutions": {
        %%RESOLUTIONS%%
    },
    "devDependencies": {
        "@theia/cli": "latest"
    }
}

And the replacement variable set to the following values.: 

%%RESOLUTIONS%%=
    "vscode-languageserver-protocol": "3.15.0-next.9",
    "vscode-languageserver-types": "3.15.0-next.5"

%%VERSION%%=latest

%%PREFERENCES%% =  
    "files.enableTrash": false
    "other.preference": "whatever"

# Assuming that recursive replacement variables are supported,
# we can maybe abstract the (latest/next) version like this:
%%$DEPENDENCIES%% = 
    "@theia/file-search": "%%VERSION%%",
    "@theia/git": "%%VERSION%%",
    "@theia/java": "%%VERSION%%",
    "@theia/json": "%%VERSION%%",
    "@theia/markers": "%%VERSION%%",
    "@theia/navigator": "%%VERSION%%",
    "@theia/outline-view": "%%VERSION%%",
    "@theia/preferences": "%%VERSION%%",
    "@theia/search-in-workspace": "%%VERSION%%",
    "@theia/terminal": "%%VERSION%%",
    "@theia/textmate-grammars": "%%VERSION%%",
    "@theia/tslint": "%%VERSION%%",
    "@theia/typescript": "%%VERSION%%",
    typescript": "latest"

# e.g. if flexibility needed we can define more categories
# of dependencies, concatenating them together in the end
%%DEPENDENCIES_EXTRA%% = 
    "@theia/docker": "%%VERSION%%",
    "someotherdep": "0.5.6"

Additional Information

• original Fix JSON language server features #272 (comment)

[akosyakov]

But it will make bad standalone examples?