feat(@schematics/angular): generate new projects with tslint 6.1.0

[alan-agius4]

tslint 6.1 adds support for TypeScript 3.8 syntax

See: https://github.com/palantir/tslint/blob/master/CHANGELOG.md#v610

Pr to update the package in the repo #17202

See individual commits for more details.

[alan-agius4]

A number of recommended rules have been removed in version 6, Should we add them in our tslint.json or not?

https://github.com/palantir/tslint/pull/4871/files and https://github.com/palantir/tslint/pull/4312/files

[dgp1130]

Discussed whether to preserve existing configs from previous version or accept new changes from TSLint.

New projects should include the new version and have a custom config with all the old rules from the previous TSLint version.

Existing projects in 9.x should have a migration allowing users to update to TSLint 6.1 if they choose to and include the old rules into their config.

In v10 we would enable the migration for everyone, need to consider how this works with Prettier as well.

This will prevent projects from backsliding on formatting. Once we get to ESLint migration, we can revisit how to best make that happen.

[cexbrayat]

@alan-agius4 Thank you for considering my proposition!
I just reviewed the updated rules and it looks good to me, except one tiny nit: variable-name has been relaxed and now allows names with a leading underscore, whereas it wasn't previously.
So to be stricly similar to what we had, I think the CLI tslint should include:

    "variable-name": {
        "options": ["ban-keywords", "check-format", "allow-pascal-case"],
    },

[alan-agius4]

@clydin PTAL

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