add {.migrated(ident, msg):... .} to issue callsite migration warnings for changed APIs

[timotheecour]

this PR adds a way to indicate an API had a change in semantics, and, when a custom defined flag is used, it will issue a new migrated warning showing both the call site and the callee symbol location, along with custom msg. eg:

proc getHomeDir*(): string {....
  migrated(nimMigratedGetHomeDir, "`getHomeDir` now does not end in DirSep, see `normalizePathEnd`").} =

import os
proc main()=
  echo getHomeDir()
main()
echo getHomeDir()

nim c main:

tests/nim/all/t12601.nim(158, 10) Warning: `getHomeDir` now does not end in DirSep, see `normalizePathEnd`; getHomeDir was migrated [proc declared in std/os(889, 6)] [Migrated]
tests/nim/all/t12601.nim(160, 8) Warning: `getHomeDir` now does not end in DirSep, see `normalizePathEnd`; getHomeDir was migrated [proc declared in std/os(889, 6)] [Migrated]

if you pass -u:nimMigratedGetHomeDir on cmdline, the warning disappears.
ditto in your user config.nims with switch("undef", "nimMigratedGetHomeDir")

you can also use --warningAsError:migrated to transform those warnings into errors, as usual

This PR (.migrated) and #18496 (--moduleoverride) address main pain points when upgrading to a new compiler version (or more generally, to a new version of a library, it's not tied to compiler changes but more generally to API changes).

future work

• allow --warning:foo:foreign:on|off to control whether to enable warnings for foreign packages; note that notes are currently turned off for foreign package notes (sane default) but in some cases it's useful to still issue those (not just for migrated warning but more generally); ditto with hints.

[stale]

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