TOML: Don't treat keys without values as errors

[techee]

While keys without = are not allowed in TOML, this situation happens all the time while editing a TOML file (until one types = after key) which produces annoying error-colorization and then re-colorization after = is inserted.

Better to allow this at the lexer level and leave the error detection job to various linters.

[nyamatongwe]

It's likely this will have to wait until after the 5.4.1 release.

[techee]

No problem.