feat: make plugin generator 3PP friendly

[mdonnalley]

What does this PR do?

Makes plugin generator 3PP friendly:

• Uses 3PP circle config (https://github.com/salesforcecli/plugin-template-sf/blob/main/.circleci/external.config.yml)
• Removes command-snapshot
• Removes git2gus
• Removes schemas
• Removes yarn scripts for testing schema and command snapshot
• Allows developer to choose a name that isn't prefixed with plugin-
• Removes @salesforce/ namespace where ever appropriate
• Removes .github

What issues does this PR fix or reference?

@W-11653212@
@W-11652178@

[cristiand391]

QA notes:

if building for a 3PP (not internal):
✅ plugin name isn't required to start with plugin-
✅ no git2gus/github, schema-related code
✅ can publish to verdaccio and install it with sf
✅ 3pp friendly circle config (no references to internal Circle context/env vars)

if building a 2PP (internal):

✅ enforces name to start with plugin-
✅ git2gus/github, schema-related code/config exists
✅ json schemas are generated

⚠️ for 3PP plugin, the files key in the pjson references the schemas folder, I tried publishing the plugin to verdaccio and didn't get a warning so not blocking but could be removed.
⚠️ what does internal mean in this context (people new to sf/sfdx plugins)? Should this be Salesforce internal team?

❌ for both cases git hooks don't work.
dev-scripts generates the husky folder and hooks but needs to run husky install to initialize it, when the plugin is created and the generator runs yarn install it makes dev-scripts bump engines.node and swallows dev-script output (it asks to run yarn install again after a modification, this one would initialize husky). PR: salesforcecli/plugin-template-sf#149

[cristiand391]

⚠️ for 3PP plugin, the files key in the pjson references the schemas folder, I tried publishing the plugin to verdaccio and didn't get a warning so not blocking but could be removed.
⚠️ what does internal mean in this context (people new to sf/sfdx plugins)? Should this be Salesforce internal team?
❌ for both cases git hooks don't work.

fixed ✅

✅ oclif.topics is updated when adding a new command (`sf dev generate command --name 'walter:white')

❌ yarn lint fails when a NUT was generated:

  28:11  error  Unsafe assignment of an `any` value             @typescript-eslint/no-unsafe-assignment
  28:20  error  Unsafe member access .stdout on an `any` value  @typescript-eslint/no-unsafe-member-access

I removed @types/shelljs here: salesforcecli/plugin-template-sf#149
the plugin shouldn't have to specify the shelljs types in its pjson anyway, this should fix it: salesforcecli/cli-plugins-testkit#274
fixed in the template ✅

❌ nyc doesn't pick up settings in .nycrc, seems it's a valid json but not for nyc.
Whatever code coverage threshold I choose, nyc always pass.

✅ generated a 3PP plugin, selected 100% code cov and generated a new command (no UT), nyc throws error about new command code with no tests.

⚠️ default nyc config (from dev-config) doesn't work, nyc always pass.
this is a known issue with dev-config.