Reduce install size

[jeddy3]

Ref: #3236

The size of the install has increased significantly (and the number of packages has nearly doubled) between 8.4.0 and 9.2.0. I believe partly because postcss-html bundles a couple of parsers:

Difference between 8.4.0 and 9.2.0:

+ stylelint@8.4.0
added 260 packages in 12.919s
jeddy3:stylelint-test jeddy3$ du -sh node_modules
 24M    node_modules

+ stylelint@9.2.0
added 418 packages in 22.232s
jeddy3:stylelint-test jeddy3$ du -sh node_modules
 33M    node_modules

For comparsion, here's ESLint:

+ eslint@4.19.1
added 141 packages in 8.82s
jeddy3:stylelint-test jeddy3$ du -sh node_modules
 19M	node_modules

Is there anything we can do about this? @gucong3000 Is it worth optimising postcss-html for <style> tags and then making markdown and template literal parsing opt-ins? Am I right in thinking it's just the parsing of <style> which is most useful for Vue and Web Component authors?

[gucong3000]

With the development of postcss-html, I changed its original design. Maybe I should find the right time to change its name. I used postcss-html for split file (not just html) for postcss syntax parsing

[gucong3000]

I believe partly because postcss-html bundles a couple of parsers

I created a new package to solve: postcss-syntax (No dependencies)

parsers are move to postcss-html postcss-markdown and postcss-styled. Some of them maybe need to move from dependencies to optionalDependencies

[jeddy3]

@gucong3000 Thanks for splitting the syntaxes into separate packages.

Once 9.2.1 is released, we can assess the new install size of stylelint. We can then discuss:

1. which syntaxes we should bundle with stylelint by default
2. which syntaxes should be used via the --custom-syntax flag

for stylelint@10.0.

[jeddy3]

I created a new package to solve: postcss-syntax (No dependencies)

I'll close this issue then.

We can move the discussion what about should be included by default to #3264