Add interval literal syntax

[Seddryck]

Summary

Add first-class interval literals to the Expressif grammar and syntax model.

Intervals should use an explicit I prefix and mathematical bound notation, while accepting both common bracket conventions.

Proposed syntax

Bounded intervals

I[1, 10]    // inclusive lower, inclusive upper
I[1, 10)    // inclusive lower, exclusive upper
I(1, 10]    // exclusive lower, inclusive upper
I(1, 10)    // exclusive lower, exclusive upper

Also accept the continental-European notation using reversed square brackets:

I[1, 10[    // inclusive lower, exclusive upper
I]1, 10]    // exclusive lower, inclusive upper
I]1, 10[    // exclusive lower, exclusive upper

The two notations are aliases and must produce the same syntax/AST semantics.

Infinite bounds

Expressif already uses +INF and -INF. They should be valid interval bounds:

I[-INF, +INF]
I[0, +INF]
I]0, +INF]
I[-INF, 0]
I[-INF, 0[

Existing sign-oriented shorthands

Preserve the established interval forms and map them to the corresponding first-class interval semantics:

I(0+)   // zero or positive
I(+)    // positive
I(0-)   // zero or negative
I(-)    // negative

Equivalent semantics:

I(0+)  == [0, +INF]
I(+)   == ]0, +INF]
I(0-)  == [-INF, 0]
I(-)   == [-INF, 0[

Dates and temporal values

Interval bounds should use the canonical Expressif literal syntax. Dates must therefore remain explicit typed literals:

I[#"2022-12-10", #"2022-12-31"[

Do not accept bare date-looking tokens such as:

I[2022-12-10, 2022-12-31]

because 2022-12-10 is otherwise lexically indistinguishable from arithmetic subtraction. The canonical date literal is #"2022-12-10".

Syntax model

The syntax/AST representation should normalize notation differences and expose at least:

• lower bound value
• upper bound value
• lower-bound inclusivity
• upper-bound inclusivity
• infinite bounds where applicable

For example, these should have identical semantics:

I[1, 10)
I[1, 10[

Acceptance criteria

• Parse bounded intervals with inclusive/exclusive endpoints.
• Accept both parenthesis and European reversed-square-bracket notation.
• Support +INF and -INF as interval bounds.
• Support the existing (0+), (+), (0-), and (-) interval concepts through the new interval literal syntax.
• Require canonical Expressif literals for temporal bounds, e.g. #"2022-12-10".
• Normalize equivalent spellings to the same syntax/AST semantics.
• Add grammar tests, syntax-node tests, and C# bindings for interval literals.