Translate syntax error messages with Fluent

[srivastava-diya]

Description

Follow up to #109 . Once the parser emits a specific mistake rather than a raw code, this issue turns that into text in the user's language.

Approach follows @hyperjump/json-schema-errors, as suggested in #84.

Scope

• A Fluent message key per mistake, with en-US translations
• Filling in whatever the parser passes alongside the mistake name
• A fallback for anything unmapped, so an unknown mistake still produces something readable
• Read the locale from the initialize params and thread it through to syntax validation

[srivastava-diya]

Now that #115 is merged, syntax diagnostics show the raw code (e.g. trailing-comma) as the message, so I believe the blocker is gone now. Here's how I plan to approach this one.

1. Messages

• Add @fluent/bundle as a direct dependency.

• Add English (en-US) messages, set up the same way as json-schema-errors, one message for each of the error codes, plus a general one for any code we don't recognise.

• Codes that carry data (number-invalid, invalid-literal, unexpected-token all pass found) use it as a Fluent variable.

• Create the bundle with useIsolating only for rtl translations, same as the recent change in json-schema-errors, so the found values don't get invisible isolation characters.

2. Localization class

• A small Localization class modelled on the one in json-schema-errors: Localization.forLocale(locale) with a cache, and one method that takes a SyntaxError and returns the message.
• The parser stays as it is. It keeps returning codes and data, and only SyntaxValidationDiagnosticsProvider turns them into text.

3. Fallbacks

• Unknown code: use unknown-message instead of throwing, so a new parser code that's missing a translation still shows something readable.
• Unsupported locale falls back to en-US or we can just throw.

4. Locale

• Read locale from the initialize params and make it available to the syntax diagnostics provider. Nothing in the server reads the locale yet, so this is new plumbing.

Tests

• A test that every SyntaxErrorCode has an en-US message, so the two can't drift apart.
• Tests for the found variable, the unknown-code fallback, and the locale fallback.

[jdesrosiers]

Sounds good. Go ahead.

[srivastava-diya]

Sounds good. Go ahead.

sure just had 1 question should the unsupported locale fall back to en-US or should we throw?

[jdesrosiers]

Yes, fallback to en-US if the locale is unsupported.