Implement Syntax 0.8

[stasm]

Let's plan the implementation work in this issue. We'll submit PRs against a release branch, zeroeight, which I'll create later today.

Some 0.8 features don't need any work in fluent.js: projectfluent/fluent#179 and projectfluent/fluent#207 are already supported and projectfluent/fluent#211 made the reference parser behave like the tooling one.

For features that need implementation, I'd like to follow the same order in which they landed in the spec. This will make it much easier to keep the reference tests green with each change. Here's the full list:

Part 1 (#307)

• 1. Support astral Unicode characters in TextElements and StringLiterals
  Support astral Unicode characters in TextElements and StringLiteral fluent#179, tests only
• 2. Don't store the term sigil in the identifier name
  Don't store the term sigil in the identifier name fluent#183, tooling parser
• 3. Treat backslash as normal char in TextElements
  Treat backslash as normal char in TextElements fluent#181, tooling parser, runtime parser
• 4. Don't normalize CRLF to LF in Junk
  Don't normalize CRLF to LF in Junk fluent#193, tests only
• 5. Forbid the closing brace in text
  Forbid the closing brace in text fluent#198, tooling parser, runtime parser

Part 2 (#309)

• 6. Dedent multiline text to preserve content indentation
  Dedent multiline text to preserve content indentation fluent#197, tooling parser, runtime parser

Part 3 (#310)

• 7. Add FunctionReference
  Add FunctionReference fluent#210, tooling parser
• 8. Store unescaped content in StringLiteral.value and raw content in StringLiteral.raw
  Store unescaped content in StringLiteral.value and raw content in StringLiteral.raw fluent#203, tooling parser

Part 4 (#311)

• 9. Add Parameterized Terms
  Add Parameterized Terms fluent#205, tooling parser, runtime parser, resolver

Part 5 (#312)

• 10. Stop parsing Junk on lines which look like Entries
  Stop parsing Junk on lines which look like Entries  fluent#211, tests only
• 11. Allow all Unicode characters
  Allow all Unicode characters fluent#207, tests only

Part 6 (#313)

• 12. Add Parameterized Attributes
  Add parameterized Attributes fluent#212, tooling parser, runtime parser, resolver

Part 7 (#314)

• 13. Recognize \UHHHHHH as an escape sequence
  Recognize \UHHHHHH as an escape sequence fluent#201, tooling parser, runtime parser
• 14. Change line endings in comments to line_end
  Change line endings in comments to line_end fluent#219, tooling parser
• 15. Forbid nested VariantLists
  Forbid nested VariantLists fluent#220, tooling parser