Support Reading Templates From Files

[MahdiBM]

In a future where the macros sandbox allows for reading from files, users could simplify this code:

import EnumeratorMacro

@Enumerator("""
enum Subtype: String {
    {{#cases}}
    case {{name}}
    {{/cases}}
}
""",
"""
var subtype: Subtype {
    switch self {
    {{#cases}}
    case .{{name}}:
        .{{name}}
    {{/cases}}
    }
}
""")
enum TestEnum {
    case a(val1: String, val2: Int)
    case b
    case testCase(testValue: String)
}

to:

import EnumeratorMacro
import EnumeratorMacroImpl

@attached(member, names: arbitrary)
macro CreateSubtype() = #externalMacro(
    module: "EnumeratorMacroImpl",
    type: "EnumeratorMacroType"
)

@CreateSubtype
enum TestEnum {
    case a(val1: String, val2: Int)
    case b
    case testCase(testValue: String)
}

And for all the next enums, you only need to use @CreateSubtype instead of writing the whole template.

How this'll work is that it'll try to read a template file which will be expected to be located at a pre-defined place like /PROJECT_ROOT/macro-templates/CreateSubtype.mustache.