Handle inline regex modifiers and invalid schema patterns in hover

[shin19991207]

What does this PR do?

• Support inline schema pattern modifiers: (?s), (?m), (?i), and their combined forms such as (?ms) / (?sm). Note that these modifiers are not part of the "portable JSON Schema regex guidance" documented at https://json-schema.org/understanding-json-schema/reference/regular_expressions, but I think supporting them is useful for schemas.
• Show diagnostic Invalid pattern: "..." when a schema uses unsupported regex syntax
• Before this fix, validation and hover failed with error Request textDocument/hover failed with message: Invalid regular expression: .... Both validation and hover should now continue to work even when schemas contain inline modifiers or unsupported patterns.

What issues does this PR fix or reference?

Fixes #874

Is it tested? How?

• New automated tests
• Tested manually

test-pattern-schema.json:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "caseInsensitiveField": {
      "type": "string",
      "description": "Field with case-insensitive pattern.",
      "pattern": "(?i)^(yes|no)$"
    },
    "multilineOnly": {
      "type": "string",
      "description": "Field with multiline pattern.",
      "pattern": "(?m)^start.*end$"
    },
    "dotallOnly": {
      "type": "string",
      "description": "Field with dotall pattern.",
      "pattern": "(?s)^start.*end$"
    },
    "multilineAndDotallCombinedFlags": {
      "type": "string",
      "description": "Field with multiline and dotall pattern.",
      "pattern": "(?ms)^start.*end$"
    },
    "dotallAndCaseInsensitiveFieldFlags": {
      "type": "string",
      "description": "Field with dotall and case-insensitive pattern.",
      "pattern": "(?s)(?i)test.*content"
    },
    "allFlags": {
      "type": "string",
      "description": "Field with case-insensitive, multiline, and dotall pattern.",
      "pattern": "(?ims)^START.*END$"
    },
    "unsupportedPattern": {
      "type": "string",
      "description": "Field with unsupported pattern.",
      "pattern": "(?x)text .*"
    }
  }
}

# yaml-language-server: $schema=./test-pattern-schema.json
caseInsensitiveField: YES

multilineOnly: |-
  ignored line
  start middle content end
  another ignored line

dotallOnly: |-
  start
  middle content
  end

multilineAndDotallCombinedFlags: |-
  ignored line
  start
  middle content
  end
  another ignored line

dotallAndCaseInsensitiveFieldFlags: |-
  TEST
  middle content
  CONTENT

allFlags: |-
  ignored line
  start
  middle content
  end
  another ignored line

unsupportedPattern: text content

[datho7561]

Seems reasonable to me.

Side node: kind of silly that ajv doesn't report if the regex is invalid when validating the schema.