doc: add documentation on ICU

[TimothyGu]

Adapted from the wiki article "Intl" by @srl295. Open to suggestions w.r.t. where this page belongs best, either here or on nodejs.org, but somewhere more permanent than a wiki.

Refs: #13644 (comment)

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

[vsemozhetbyt]

If it remains in the api, it should also be added to all.md

[vsemozhetbyt]

Linter CI: https://ci.nodejs.org/job/node-test-linter/10073/

[jasnell]

Please avoid use of informal pronouns like we in the docs... Also, this last sentence is a bit off for me... Perhaps:

Because it is expected that most Node.js users will make use of only a small portion
of ICU functionality, only a subset of the full ICU data set is provided by Node.js by
default. Several options are provided for customizing and expanding the ICU data set
either when building or running Node.js.

[jasnell]

Please avoid using informal pronouns like your in the docs.

[jasnell]

As above, please avoid using informal pronouns like you

[TimothyGu]

@jasnell Comments are addressed. What are your thoughts on where this document should end up?

[TimothyGu]

I'll merge this tomorrow if there are no more comments.

[vsemozhetbyt]

Linter (you may need to rebase after the 15599cb):

 96:1 error Expected indentation of 0 spaces but found 2 indent
98:1 error Expected indentation of 0 spaces but found 2 indent
99:1 error Expected indentation of 0 spaces but found 2 indent

[TimothyGu]

Not too big of a fan of the linter change, but changed.

Linter CI: https://ci.nodejs.org/job/node-test-linter/10274/

[vsemozhetbyt]

@TimothyGu It seems it is due to 7477f9b, but I wanted to be on safe side with updated tools)

[TimothyGu]

Landed in 6809429

[MylesBorins]

Is this applicable to v6.x? It is easy enough to land this but I want to ensure that it makes sense for the version of ICU that is embedded

6.x: 58.2
8.x: 59.1

[TimothyGu]

@MylesBorins Yep, most of the content is applicable to 6.x as well, but some things need to be changed (e.g. mentions of the WHATWG URL API as it doesn't exist in 6.x).

[srl295]

post merge 👍