Fix some issues with ExprParse

[UnderscoreTud]

Description

This PR fixes a couple issues with ExprParse:

• Previously, you could obtain an array object when parsing text with a plural type. For example, broadcast "1, 2" parsed as "%ints%" # broadcasts [1, 2]. Now, it flattens the array if you're not setting a list to the expression.
• Previously, you could encounter an UnparsedLiteral exception if you tried to parse text as %object%. I fixed it by checking whether the types used have a parser or not. If one of them doesn't, it errors.

Additionally, this PR introduces a new ParseContext used specifically for this expression, called PARSE.

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Target Minecraft Versions: any
Requirements: none
Related Issues: none