…Luau::toString (#1889)

## General
- Support AstStatDeclareGlobal output as a source string (via
@karl-police in #1889)
- Luau heap dump correctly reports the size of a string, now including
overhead for the string type
- Prevent yields from Luau `xpcall` error handling function.
 
## Analysis
- Avoid exponential blowup when normalizing union of normalized free
variables.
- Fix type pack-related bugs that caused infinite recursion when:
  - A generic type pack was bound to itself during subtyping.
- In type pack flattening, when that same generic type pack was now
being bound another generic type pack which contained it.
- Properly simplify `any & (*error-type* | string)` to `*error-type* |
*error-type* | string` instead of hanging due to creating a huge union
type.

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Co-authored-by: Andy Friesen <afriesen@roblox.com>
Co-authored-by: Ariel Weiss <aaronweiss@roblox.com>
Co-authored-by: Hunter Goldstein <hgoldstein@roblox.com>
Co-authored-by: Sora Kanosue <skanosue@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>

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Co-authored-by: Hunter Goldstein <hgoldstein@roblox.com>
Co-authored-by: Varun Saini <61795485+vrn-sn@users.noreply.github.com>
Co-authored-by: Menarul Alam <malam@roblox.com>
Co-authored-by: Aviral Goel <agoel@roblox.com>
Co-authored-by: Vighnesh <vvijay@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>
Co-authored-by: Ariel Weiss <aaronweiss@roblox.com>
Co-authored-by: Andy Friesen <afriesen@roblox.com>