Split spec/registry.md into multiple files under spec/functions/ (#974)

* Move spec/registry.md -> spec/functions/README.md

* spec/functions: Split out datetime.md, number.md & string.md + add TOC to README.md

Author: eemeli

spec/functions/README.md

@@ -0,0 +1,87 @@
+# MessageFormat 2.0 Default Function Registry
+
+## Table of Contents
+
+1. [Introduction](#introduction)
+1. [String Value Selection and Formatting](string.md)
+   1. [`:string`](string.md#the-string-function)
+1. [Numeric Value Selection and Formatting](number.md)
+   1. [`:number`](number.md#the-number-function)
+   1. [`:integer`](number.md#the-integer-function)
+   1. [`:math`](number.md#the-math-function)
+   1. [`:currency`](number.md#the-currency-function)
+   1. [`:unit`](number.md#the-unit-function)
+1. [Date and Time Value Formatting](datetime.md)
+   1. [`:datetime`](datetime.md#the-datetime-function)
+   1. [`:date`](datetime.md#the-date-function)
+   1. [`:time`](datetime.md#the-time-function)
+
+## Introduction
+
+This section defines the **REQUIRED** _functions_ which are REQUIRED for conformance with this specification,
+along with **RECOMMENDED** _functions_ that SHOULD be implemented to support
+additional functionality.
+
+To **_<dfn>accept</dfn>_** a function means that an implementation MUST NOT
+emit an _Unknown Function_ error for that _function_'s _identifier_.
+To _accept_ an _option_ means that a _function handler_ MUST NOT
+emit a _Bad Option_ error for that _option_'s _identifier_ when used with the _function_
+it is defined for
+and MUST NOT emit a _Bad Option_ error for any of the _option_ values
+defined for that _option_.
+Accepting a _function_ or its _options_ does not mean that a particular output is produced.
+Implementations MAY emit an _Unsupported Operation_ error for _options_
+or _option_ values that they cannot support.
+
+_Functions_ can define _options_. 
+An _option_ can be **REQUIRED** or **RECOMMENDED**.
+
+Implementations MUST _accept_ each **REQUIRED** _function_ and
+MUST _accept_ all _options_ defined as **REQUIRED** for those _functions_.
+
+Implementations SHOULD _accept_ each **RECOMMENDED** _function_. 
+For each such _function_, the implementation MUST accept all _options_
+listed as **REQUIRED** for that _function_.
+
+Implementations SHOULD _accept_ _options_ that are marked as **RECOMMENDED**.
+
+Implementations MAY _accept_ _functions_ not defined in this specification.
+In addition, implementations SHOULD provide mechanisms for users to
+register and use user-defined _functions_ and their associated _functional handlers_.
+Functions not defined by any version of this specification SHOULD use 
+an implementation-defined or user-defined _namespace_.
+
+Implementations MAY implement additional _options_ not defined
+by any version of this specification
+for **REQUIRED** and **RECOMMENDED** functions.
+Such _options_ MUST use an implementation-specific _namespace_.
+
+Implementations MAY _accept_, for _options_ defined in this specification,
+_option_ values which are not defined in this specification.
+However, such values might become defined with a different meaning in the future,
+including with a different, incompatible name
+or using an incompatible value space.
+Supporting implementation-specific _option_ values for **REQUIRED** or **RECOMMENDED** functions is NOT RECOMMENDED.
+
+Implementations MAY _accept_, for _operands_ or _options_ defined in this specification,
+values with implementation-defined types.
+Such values can be useful to users in cases where local usage and support exists
+(including cases in which details vary from those defined by Unicode and CLDR).
+
+> For example:
+> - Implementations are encouraged to _accept_ some native representation
+>   for currency amounts as the _operand_ in the _function_ `:currency`.
+> - A Java implementation might _accept_ a `java.time.chrono.Chronology` object
+>   as a value for the _date/time override option_ `calendar`
+> - ICU4J's implementation might _accept_ a `com.ibm.icu.text.NumberingSystem` object
+>   instead of using a [Unicode Numbering System Identifier](https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#UnicodeNumberSystemIdentifier)
+>   for the option `numberingSystem` in _functions_ such as `:number` or `:integer`.
+
+Future versions of this specification MAY define additional _options_ and _option_ values,
+subject to the rules in the [Stability Policy](#stability-policy),
+for _functions_ found in this specification.
+As implementations are permitted to ignore _options_ that they do not support,
+it is possible to write _messages_ using _options_ not defined below
+which currently format with no error, but which could produce errors
+when formatted with a later edition of this specification.
+Therefore, using _options_ not explicitly defined here is NOT RECOMMENDED.