Context key scanner rejects the d and v regex flags in when clauses

[VaggelisGian]

Bug description

The context key expression scanner whitelists which regex flags may follow a regex literal in a when clause (src/vs/platform/contextkey/common/scanner.ts, _regexFlags). The set is

['i', 'g', 's', 'm', 'y', 'u']

which is missing the two newer ECMAScript flags: d (hasIndices) and v (unicodeSets). Both are valid in the engines VS Code ships on, but a when clause that uses them fails to lex: the trailing flag character is not consumed as part of the regex token, becomes a separate Str token, the parser then reports "Expected: REGEX", and ContextKeyExpr.deserialize returns undefined for a perfectly legal clause.

Example: an extension manifest containing

"when": "a =~ /x/d"

silently evaluates to never-match today.

Steps to reproduce

1. Add a keybinding or view with "when": "editorTextFocus =~ /x/d" (any regex using the d or v flag).
2. Observe the clause never activates; enabling context key tracing shows it deserialized to undefined.

Flags from the whitelist (/x/i, /x/gm) work as expected.

Expected behavior

All ECMAScript regex flags accepted by the runtime should lex, parse and deserialize, so /x/d and /x/v clauses behave like /x/i.

Version tested

Commit 77f86f3d3a0 on main. Reproduced at the unit level: scanning 'foo =~ /zee/d' yields a trailing Str 'd' token instead of a single RegexStr '/zee/d'. A fix plus scanner tests and an end-to-end deserialize test is ready.